United States
Environmental Protection
Agency
Office of
Research and Development
Washington, DC 20460
EPA/625/6-91/026
August 1991
ปEPA Handbook
Stabilization
Technologies for RGRA
Corrective Actions
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EPA/625/6-91/026
August 1991
Handbook
Stabilization Technologies
for RCRA Corrective Actions
August 1991
Prepared for:
U.S. Environmental Protection Agency
Office of Research and Development
/ Center for Environmental Research Information
-Cincinnati, OH 45268
Prepared by:
Metcalf & Eddy, Inc.
Wakefield, MA
Printed on Recycled Paper
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NOTICE
i
This document is intended to assist regional, state, and facility personnel in identifying, evaluating,
and implementing stabilization technologies. However, EPA will not necessarily limit stabilization tech-
nology evaluations to those that comport with the guidance set forth herein. This document is not a
regulation (i.e., it does not establish a standard pf conduct that has the force of law) and should not be
used as such. Regional, state, and facility personnel must exercise their discretion in using this guid-
ance document as well as other relevant information in determining whether interim or final stabiliza-
tion corrective actions meet the regulatory standard.
Mention of trade names or commercial products does not constitute endorsement or recommendation
for use by the U.S. Environmental Protection Ag|ency.
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ACKNOWLEDGMENTS
This document is a compilation of material applicable to RCRA stabilization actions. Mr. Willard
Baker of Metcalf & Eddy, Inc., Wakefield, Massachusetts, and Ms. Heidi Schultz of Eastern Re-
search Group, Inc., Arlington,,Massachusetts, provided project management. Mr. Edwin Barth of
the Center for Environmental Research Information, Cincinnati, Ohio, provided overall technical
direction for the project. Editorial and production assistance was provided by Karen Ellzey, Susan
Richmond, and Denise Short of ERG.
Development of Material
Stabilization Technology Implementation
Ms. Polly Whitmore, P.E., Metcalf & Eddy, Inc., Wakefield, Massachusetts
Containment
Mr. Nicholas C. D'Agostino, P.E., Metcalf & Eddy, Inc., Wakefield, Massachusetts
Mr. Warren Diesl, C.G., Metcalf & Eddy, Inc., Wakefield, Massachusetts
Soils Treatment
Mr. Karl DeBisschop, Metcalf & Eddy, Inc., Wakefield, Massachusetts
Ms. Diane Dopkin, Metcalf & Eddy, Inc., Wakefield, Massachusetts
Ms. C'inthia McLane, P.E., Metcalf & Eddy, Inc., Wakefield, Massachusetts
Ground-Water Treatment
Mr. Warren Diesl, C.G., Metcalf & Eddy, Inc., Wakefield, Massachusetts
Mr. Donald Dwight, P.E., Metcalf & Eddy, Inc., Wakefield, Massachusetts
Ms. Cinthia McLane, P.E., Metcalf & Eddy, Inc., Wakefield, Massachusetts
Technical Reviewers
Mr. Edwin F. Barth, P.E., U.S. EPA, Center for Environmental Research Information,
Cincinnati, Ohio
Dr. Milovan S. Beljin, University of Cincinnati, Cincinnati, Ohio
Mr. John E. Matthews, U.S. EPA, R.S. Kerr Environmental Research Laboratory, Ada, Oklahoma
Dr. Lawrence C. Murdock, University of Cincinnati, Center Hill Solid and Hazardous Waste
Research Facility
Mr. Vernon Myers, U.S. EPA, Office of Solid Waste, Washington, D.C.
Mr. Jon Perry, U.S. EPA, Office of Solid Waste, Washington, D.C.
Mr. Reid Rosnick, U.S. EPA, Office of Solid Waste, Washington, D.C.
Mr. Bill Stelz, U.S. EPA, Office of Solid Waste, Washington, D.C.
Dr. Robert L. Siegrist, P.E., Oak Ridge National Laboratory Environmental Science Division
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PREFACE
On Novembers, 1984,'Congress enacted the Hazardous and Solid Waste Amendments (HSWA) to
the Resource Conservation and Recovery Act (RCRA). RCRA requires a corrective action program
that prevents hazardous constituents from exceeding concentration limits at the compliance point
(i.e., the boundary of the waste management area) if any contaminant level exceeds the ground-
water, surface water, air, and soil protection standards. Among the most significant provisions of
HSWA are ง3004(u), which requires corrective action for releases of hazardous waste or constitu-
ents from solid waste management units (SWMUs) at hazardous waste treatment, storage, and dis-
posal facilities seeking final RCRA permits; and ง3004(v), which compels corrective action for
releases that have migrated beyond the facility property boundary. Under Subpart S of the RCRA
regulations, requirements for corrective action at SWMUs are currently being drafted. The final Sub-
part S rule will establish the basic direction anid goals for the program.
The overall goal of the RCRA corrective action program stabilization initiative is to, as soon as possi-
ble, control or abate imminent threats to human health and the environment from releases from
RCRA facilities, and to prevent or minimize, the further spread of contamination while long-term
remedies at facilities are pursued. Using sound engineering judgment, stabilization actions, imple-
mented under the interim measures authority of the RCRA corrective action program, could achieve
rapid source control, stabilization, containment, or other results to significantly reduce the severity of
a problem at a site. By setting priorities based on the environmental severity of sites, the U S Envi-
ronmental Protection Agency (EPA) plans to include more sampling appropriate to technology selec-
tion during the site investigation phase of the RCRA correction action process to allow development
of specific corrective action provisions in permits and orders, particularly for interim measures.
Guidance is provided herein on identification [of the types of environmental settings that should be
the focus of stabilization actions, on technical approaches to accelerate data gathering in support of
decisions on appropriate stabilization measures, and on phasing the RCRA Facility Investigation
process to gather the necessary data to make timely decisions within the framework of the existinq
corrective action program.
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LIST OF ACRONYMS AND ABBREVIATIONS
AFB Air Force Base
API American Petroleum Institute
ASTM American Society for Testing and Materials
ATM Atmosphere
BOD Biochemical Oxygen Demand
CAP Corrective Action Plan
CB Cement-Bentonite
CERCLA Comprehensive Environmental Response, Compensation, and Liability Act
cfm Cubic Feet per Minute
CFR Code of Federal Regulations
CM Corrective Measures
CMI Corrective Measures Implementation
CMS Corrective Measures Study
COD Chemical Oxygen Demand
DNAPL Dense Nonaqueous Phase Liquid
DO Dissolved Oxygen
EP Tox Extraction Procedure Toxicity Test
EPA U.S. Environmental Protection Agency
EPI Environmental Priorities Initiative ;
FEMA Federal Emergency Management Agency
HOPE High Density Polyethylene
HEA Health and Environmental Assessment
HELP Hydrologic Evaluation of Landfill Performance
HHE Human Health and the Environment
HSWA Hazardous and Solid Waste Amendments (RCRA)
LDR Land Disposal Restriction
LNAPL Light Nonaqueous Phase Liquid
m3/m-min Cubic meter per meter-minute
MCLs Maximum Contaminant Levels
NAPL Nonaqueous Phase Liquids
NFIP National Flood Insurance Program
NOAA National Oceanic and Atmospheric Administration
NPDES National Pollutant Discharge Elimination System
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OD
OSWER
PAH
PCS
pcf
ppb
ppm
psi
PVC
QA/QC
RCRA
RFA
RO
RFI
ROD
SB
SCFM/ft
SCS
SDWA
SITE
SPT
S/S
SVOC
SWMU
TCLP
TOC
USGS
UST
UV
VOC
VLDPE
Outer Diameter
Office of Solid Waste and Emergency Response
Polynuclear Aromatic Hydrocarbon
Polychlorinated Biphenyl
Pounds per Cubic Foot
Parts per Billion
Parts per Million
Pounds per square inch
Polyvinyl Chloride
Quality Assurance/Quality Control
Resource Conservation and Recovery Act
RCRA Facility Assessment
Reverse Osmosis
RCRA Facility Investigation
Rock Quality Designation
Soil-Bentonite
Standard Cubic Feet per Minute per Foot
Soil Conservation Service
Safe Drinking Water Act
Superfund Innovative Technology Evaluation
Standard Penetration Test
Solidification/Stabilization
Semivolatile Organic Compound
Solid Waste Management Unit
Toxicity Characteristic Leaching Procedure
Total Organic Carbon :
United States Geologic Survey
Underground Storage Tank
Ultraviolet
Volatile Organic Compound
Very Low Density Polyethylene
Mlcrograms per Liter
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CONTENTS
Page
1 INTRODUCTION 1
1.1 BACKGROUND --1
1.2 RCRA STABILIZATION STRATEGY 1
1.3 CORRECTIVE ACTION PROCESS : 2
1.4 IMPLEMENTATION OF STABILIZATION MEASURES 3
1.5 IDENTIFICATION AND SCREENING OF STABILIZATION TECHNOLOGIES 3
1.6 DOCUMENT ORGANIZATION : 6
1.7 REFERENCES 7
2 CONTAINMENT TECHNOLOGIES 9
2.1 DRAINS/TRENCHES - - 9
2.1.1 Data Collection Requirements 9
2.1.2 Evaluation 10
2.1.3 Engineering Considerations for Implementation 12
2.2 VERTICAL WELLS 12
2.2.1 Data Collection Requirements 12
2.2.2 Evaluation 12
2.2.3 Engineering Considerations for Implementation 13
2.3 HORIZONTAL WELLS - 13
2.3.1 Data Collection Requirements 13
2.3.2 Evaluation 14
2.3.3 Engineering Considerations for Implementation ..., - 14
2.4 SLURRY CUTOFF TRENCH/WALL : - 14
2.4.1 Data Collection Requirements 16 *
2.4.2 Evaluation 16
2.4.3 Engineering Considerations for Implementation 16
2.4.3.1 Soil-Bentonite Slurry Cutoff Trench/Wall 16
2.4.3.2 Cement-Bentonite Slurry Cutoff Trench/Wall : - 18
2.5 SHEET PILE CUTOFF WALL 18
2.5.1 Data Collection Requirements 19
2.5.2 Evaluation 19
2.5.3 Engineering Considerations for Implementation 19
2.6 GROUTING... # 19
2.6.1 Data Collection Requirements 19
2.6.2 Evaluation 19
2.6.3 Engineering Considerations for Implementation 20
VII
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2.7 CAPPING , t .; 24
2.7.1 Data Collection Requirements 24
2.7.2 Evaluation ; , 24
2.7.3 Engineering Considerations for Implementation 24
2.8 SURFACE WATER CONTROL METHODS ..28
2.8.1 Data Collection Requirements j 28
2.8.2 Evaluation J 28
2.8.3 Engineering Considerations for Implementation..... 28
2.9 GAS VENTING ! 29
2.9.1 Data Collection Requirements 30
2.9.2 Evaluation ..30
2.9.3 Engineering Considerations for Implementation 31
2.10 HYDRAULIC FRACTURING 32
2.10.1 Data Collection Requirements : 32
2.10.2 Evaluation I 32
2.10.3 Engineering Considerations for Implementation .- '. 32
2.11 REFERENCES 33
3 SOILS TREATMENT TECHNOLOGIES 35
3.1 SOLIDIFICATION/STABILIZATION ; . 35
3.1.1 Data Collection Requirements , 37
3.1.2 Evaluation 4 ....37
3.1.3 Engineering Considerations for Implementation ..39
3.2 SOIL FLUSHING L..... 40
3.2.1 Data Collection Requirements 41
3.2.2 Evaluation [ 41
3.2.2.1 Treatability Testing ...i .41
3.2.3 Engineering Considerations for Implementation 41
3.2.3.1 Introduction and Recovery of the Flushing Solution 41
3.2.3.2 Hydraulic Controls : 41
3.2.3.3 Ground-water/Flushing Solution Treatment. 41
3.2.3.4 System Performance j . 41
3.3 BIOREMEDIATION ; 42
3.3.1 Data Collection Requirements I 43
3.3.2 Evaluation ..43
3.3.2.1 Treatability Testing '. 44
3.3.2.2 Mathematical Models 45
3.3.2.3 Field Operation ....;... 45
3.3.3 Engineering Considerations for Implementation 45
3.3.3.1 Enhancement of Biological|Mechanisms 47
3.3.3.2 Ancillary Equipment 47
3.4 VACUUM EXTRACTION ,. 43
3.4.1 Data Collection Requirements 43
3.4.2 Evaluation .r...... 43
3.4.3 Engineering Considerations for Implementation 49
3.5 REFERENCES 53
vw
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WATER TREATMENT TECHNOLOGIES 55
4.1 PUMP-AND-TREAT " 55
4.1.1 Data Collection Requirements 56
4.1.2 Evaluation 56
4.1.3 Engineering Considerations for Implementation ." 55
4.2 GROUND-WATER TREATMENT OPTIONS 61
4.3 REFERENCES 61
IX
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TABLES
No.
1-1
1-2
2-1
2-2
2-3
2-4
2-5
2-6
2-7
2-8
2-9
2-10
2-11
2-12
3-1
3-2
3-3
3-4
3-5
3-6
4-1
Title Pa9e
Examples of Interim (Stabilization) Measures 4
Soil and Ground -Water Stabilization 6
Data Requirements for Drains/Trenches ." 11
Soil-Benton'rte Permeability Increases Due to Leaching with Various Pollutants 15
Data Requirements for Slurry Cutoff Trench/Wall 17
Data Requirements for Sheet Pile Cutoff Wall 20
Interactions between Grouts and Generic Chemical Classes 21
Interactions between Grouts and Specific Chemical Classes 22
Data Requirements for Grouting 23
Data Requirements for Capping..:. : 25
Quality Control Requirements for Soil-Bentonite Caps 27
Data Requirements for Runoff/Run-on Control 29
Data Requirements for Gas Venting 31
Data Requirements for Hydraulic Fracturing 33
Data Requirements for Solidification/Stabilization Technology 38
Data Requirements for Soil Flushing and Chefnical Extraction Systems 42
Critical Environmental Factors for Microbial Activity 44
Site and Soil Characteristics for In Situ Treatment 45
Data Requirements for Bioremediation 46
Data Requirements for Vacuum Extraction 50
Data Requirements for Pump-and-Treat Systems 57
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FIGURES
No. Description Page
2-1 Applicability of Containment Technologies to Site Conditions 10
2-2 Schematic Diagram of Horizontal Drilling Well System 13
2-3 Typical Slurry Wall Construction Site 18
*
2-4 Soil-Bentonite Slurry Cutoff Trench Cross Section 18
2-5 Soil Gradation versus Grout Type 21
2-6 Typical Grout Pipe Layout for Grout Curtain 21
2-7 Soil Permeability versus Vapor Flowrate for Several Values of Applied Vacuum 30
3-1 Applicability, of Soils Treatment Technologies 36
3-2 Schematic of a Vacuum Extraction System 52
3-3 Typical Extraction/Air Inlet Well Construction 52
4-1 Flow Chart to Determine the Level of Modeling Required 60
4-2 Applicability of Treatment Technologies to Contaminated Ground Water 62
XI
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CHAPTER ONE
Introduction
The overall goal of implementing stabilization technolo-
gies within the existing corrective action process is to
control or abate imminent threats to human health and
the environment from releases at RCRA facilities. The in-
tent is to prevent or minimize the further migration of con-
taminants while long-term corrective action remedies are
evaluated and implemented. Although stabilization meas-
ures may be applied to address releases to any medium
(soil, ground water, surface water, air), the U.S. Environ-
mental Protection Agency (EPA) anticipates that, as a
practical matter, a large proportion of stabilization actions
will focus on soil and ground-water media to prevent fur-
ther degradation. This document emphasizes the contain-
ment of contamination and the treatment of contaminated
soils. The treatment of contaminated ground water is
briefly discussed with references to the many previous
publications pertaining to the screening of ground-water
treatment options.
1.1 BACKGROUND
The Hazardous and Solid Waste Amendments (HSWA)
to the Resource Conservation and Recovery Act (RCRA)
were enacted into law on November 8, 1984. One of the
major provisions (Section 3004(u)) of these amendments
requires corrective action for releases of hazardous
waste or constituents from solid waste management units
(SWMUs) at hazardous waste treatment, storage, or dis-
posal facilities. Under this provision, any facility applying
for a RCRA hazardous waste management facility permit
will be subject to a RCRA Facility Assessment (RFA).
The RFA is conducted by the regulatory agency and is
designed to identify SWMUs which are, or are suspected
to be, the source of a release to the environment. If any
such units are identified, the owner or operator of the fa-
cility will be directed to perform a RCRA Facility Investi-
gation (RFI) to obtain information on the nature and
extent of the release so that the need for interim correc-
tive measures or a Corrective Measures Study (CMS)
can be determined. Information collected during the RFI
can also be used by the owner or operator to aid in for-
mulating and implementing appropriate corrective meas-
ures. Such corrective measures may range from stopping
the release through the application of a source control
technique to a full-scale cleanup of the affected area. In
cases where releases are sufficiently characterized, the
regulatory agency may require the owner or operator to
collect specific information needed to implement correc-
tive measures during the RFI (U.S. EPA, 1989a).
Approximately 5,700 Subtitle C facilities, including over
80,000 SWMUs, are regulated by Section 3004 of HSWA to
implement corrective actions for the release of hazardous
waste. When federal facilities are considered, the scope of
the problem is enlarged. To speed up the implementa-
tion of corrective actions, the "RCRA Implementation
Study" recommended a "stabilization" approach. This ap-
proach would control or limit further releases without delay
when an immediate threat exists or there is an opportunity
to get action underway quickly (U.S. EPA, 1990b).
1.2 RCRA STABILIZATION STRATEGY
The intent of stabilization is to prevent or minimize the
further migration of contaminants due to releases at
RCRA facilities. This minimizes the complexity, and
therefore, the cost, of future corrective actions when
long-term corrective action remedies are to be evaluated
and implemented. To streamline the process, EPA envi-
sions stabilization measures will be identified and imple-
mented under the interim measure authority, concurrently
with the ongoing phases of the RFA and RFI activities.
Implementation of the stabilization initiative will generate
substantial costs for RCRA facility owners and operators,
but will also create substantial environmental benefits, as
well as potential future cost benefits by taking earlier ac-
tion. The Environmental Priorities Initiative (EPl) is an in-
tegrated RCRA/Superfund effort used to identify and
evaluate contaminated sites that present the greatest risk
to human health and the environment. Focusing re-
sources in the near-term on stabilizing environmental
problems at the highest priority facilities, rather than pur-
suing lengthy investigations culminating in final compre-
hensive remedies at fewer facilities, should enable EPA
and states to quickly control the most serious environ-
mental problems at a greater number of facilities. Fur-
thermore, by imposing such controls more expeditiously,
the extent and incidence of continued environmental deg-
radation from existing releases should be significantly re-
duced. These actions will result in a reduction of future
facility liability, including potential contaminant exposure.
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Although stabilization measures may be applied to ad-
dress releases to any medium (soil, ground water, sur-
face water, air), EPA anticipates that, as a practical
matter, a large proportion of stabilization actions will fo-
cus on soil and ground-water media to prevent further
degradation. EPA experience with the corrective action
program has shown that a substantial number of RCRA
facilities have caused ground-water contamination from
past waste management practices. Contaminated aqui-
fers may be a serious health risk when used as a drinking
water supply. Migration of contaminants can also result (in
serious ecological damage to both surface waters and
wetlands. In addition, there is growing evidence regard-
ing the technical limitations of restoring contaminated
ground water to health and drinking water standards.
This further underscores the need to focus remedial ef-
forts on the prevention of new contamination as well as
preventing further migration of existing contamination. !
With the implementation of stabilization measures at
RCRA facilities, EPA may be able to limit active oversight
at low-risk facilities for some period of time in order to ad-
dress other high-priority facilities. In other circumstances,
stabilization could simply be a milestone within the reme-
diation process. There may also be a limited number of
situations where a stabilization measure could be consid-
ered to be the "final" remedy for a facility.
1.3 CORRECTIVE ACTION PROCESS
The RCRA corrective action process was established tb
investigate and implement cleanup for releases of haz-
ardous wastes and/or constituents to the environment at
facilities through permit conditions or by an administrative
order or judicial action. As of this writing, the draft RCRA
corrective action regulations (40 CFR 264 Subpart S) de-
scribing many procedural aspects of the RCRA corrective
action process have not been published. However, sev-
eral documents provide the framework for directing the
development of the site-specific work to be performed by
the owner/operator in the facility's corrective action pro,-
gram: the RCRA Facility Investigation Guidance Interim
Rnal (U.S. EPA, 1989a), RCRA Corrective Action Interim
MeasuresInterim Final (U.S. EPA, 1988a), and the
RCRA Corrective Action PlanInterim Final (U.S. EPA,
1988c). From an engineering perspective, these docuT
ments provide scopes of work that can assist owner/op-
erators and engineers in identifying each phase of a
facility-wide corrective action plan, and in the formulation
and implementation of interim measures, respectively
(U.S. EPA, 1989b).
Basic activities common to the corrective action process
follow:
ป RCRA Facility Assessment (RFA) - Systematic iden-
tification of actual or potential releases through exami-
nation of each solid waste management unit
RCRA Facility Investigation (RFI) - Characterization
of the nature, extent, and rate of migration of each re-
lease, and preinvestigation identification of possible
containment/treatment technologies
Corrective Measures Study (CMS) - Identification of
appropriate corrective measures, and detailed feasibil-
ity evaluation of cleanup alternatives
Corrective Measures Implementation (CMI) - De-
sign, construction, and implementation of corrective
measures including performance monitoring
Interim measures are defined as corrective actions to
stabilize, control, or limit further releases and can be im-
plemented at any point in the process where there is an
immediate threat to human health or the environment.
Health and Environmental Assessments (HEA) are
performed concurrently with the corrective action process
to identify exposure and action levels for corrective
measures. The HEA is a continuous process that begins
with the initiation of the RFI. As investigation data (from
monitoring and/or modeling) become available, both dur-
ing and at the conclusion of discrete phases, the regula-
tory agency should be notified. The regulatory agency
compares these data to applicable health and environ-
mental criteria, including an evaluation against qualitative
criteria, to determine the need for (1) a stabilization cor-
rective action under the authority of an interim meas-
ure(s) and/or (2) a CMS.
The regulatory agency may require both interim (stabili-
zation) measures and a CMS as a result of the HEA. The
difference between interim stabilization measures and the
final corrective action is timing. The development and im-
plementation of a comprehensive corrective action plan is
often a time-consuming process. Between the time of the
identification of a contaminant release and the implementa-
tion of corrective measures, existing conditions or further
contaminant migration could endanger human health and
the environment.
Specific health and environmental criteria and proce-
dures to conduct the HEA for interim corrective measures
are discussed in the Interim Final RFI Guidance (U.S.
EPA, 1989a). The health and environmental criteria pro-
vided in the guidance do not necessarily represent target
cleanup levels that must be achieved through the imple-
mentation of corrective measures. Rather, they establish
presumptive levels that indicate that a closer examination
is necessary. This closer analysis would generally take
place as part of a CMS. The owner or operator has a
continuing responsibility to identify and respond to emer-
gency situations and to define priority situations that may
warrant interim corrective measures. For these situations,
the owner or operator should follow the RCRA Contin-
gency Plan required under 40 CFR Part 264, Subpart D
and Part 265, Subpart D.
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All steps except the RFA and HEA are conducted by the
owner/operator of the facility, with oversight by EPA or
state agency; the RFA and HEA are conducted directly
by EPA or the state. The steps, which are discussed in
detail in the guidance, do not need to be followed rigidly
by the EPA Regions. For example, the CMS can be
modified or eliminated if a single remedial alternative is
obvious.
1.4 IMPLEMENTATION OF STABILIZATION
MEASURES
EPA has established a key priority siting process to
stabilize releases from the most environmentally sensitive
facilities.
The Environmental Priorities Initiative (EPI) is an inte-
grated RCRA/Superfund effort to identify and evaluate
sites that present the greatest risk to human health and
the environment. EPA's prioritization of sites to determine
whether a facility is an appropriate candidate for interim
stabilization measures is based on a number of factors.
Among the factors considered in deciding the need for
stabilization measures are the severity of environmental
problems at the site, the site's complexity and compli-
ance history, the level of public involvement, and the
availability of resources.
Site history and characterization data may be available
before the RFA has been completed, especially data on
imminent threats. However, in most situations, data on
the fate and transpprt of hazardous constituents will not
be available until the RFI is underway or completed. To
streamline the process, it is clear that data needed to
make decisions on stabilization should be gathered early
in the RFI process.
The amount of information needed to support technical
decisions for stabilization will vary greatly. Table 1-1 pro-
vides a list of possible stabilization measures for various
units and release types. This list is not considered to be
all inclusive. More information is available through the
RCRA Corrective Action Interim Measures Guidance - In-
terim Final (U.S. EPA, 1988a). Obvious removal-type
situations might often require some information to de-
termine the quantity and location of wastes, after which
removal may be done more or less immediately, with-
out extensive studies. However, ground-water con-
tamination in a complex hydrogeologic setting could
require an extensive facility investigation where existing
site characterization is limited. This would allow devel-
opment of specific corrective action provisions in per-
mits and orders, before an effective stabilization remedy
could be implemented.
1.5 IDENTIFICATION AND SCREENING OF
STABILIZATION TECHNOLOGIES
The identification of stabilization technologies may be an
ongoing process, conducted in phases as data become
available. Each site provides a unique set of constraints.
As more site data become available, work completed on
earlier tasks may need to be revisited and supplemented
to address the newly apparent physical and chemical
conditions. The screening and selection of stabilization
technologies and the decision for remedial action are
usually determined from the sequenced tasks below:
Identify initial stabilization remediation goals. These
goals may be refined and revised during design, or
later during startup and operation.
Identify potential stabilization technology applications.
Screen technologies using available data.
Obtain additional data, as necessary, to evaluate tech-
nology application further.
Design, operate, and monitor technology application.
Other EPA documents have been written to assist techni-
cal decision-makers with the screening of corrective ac-
tion technologies. The Risk Reduction Engineering
Laboratory published a document to assist users in mak-
ing preliminary evaluations of the technologies that might
be employed to remediate a petroleum release (U.S.
EPA 1990a). The Center for Environmental Research In-
formation and the Office of Solid Waste and Emergency
Response have published documents that provide infor-
mation on the identification, selection, and application of
technologies suitable for controlling and treating hazard-
ous waste releases from RCRA facilities (U.S. EPA,
1989b, U.S. EPA, 1988d). The Office of Solid Waste and
Emergency Response published a reference guide that
profiled various emerging and innovative technologies
under the Superfund Innovative Technology Evaluation
(SITE) program (U.S. EPA, 1990c). Other documents dis-
cuss in situ bioremediation (U.S. EPA, 1990d), and pro-
vide the latest information on many in situ treatments of
hazardous waste (U.S. EPA, 1990e).
RFI activities conducted at several facilities that have im-
plemented RCRA stabilization actions were reviewed to
obtain input for the preparation of this manual. Often, it
was found that interim measures were selected and im-
plemented in stages during the RFI process as data were
collected that indicated the need for a particular stabiliza-
tion action. In some cases, stabilization measures taken
were strictly of an interim nature, to prevent the migration
of contamination, but not necessarily to provide the only
corrective action. Other stabilization actions were taken
to prevent contaminant migration and also to provide the
degree of treatment necessary for site remediation. In the
latter case, operational and post-operational performance
monitoring provide important feedback for an effective cor-
rective action program.
Successful stabilization actions provide flexibility in the
corrective action process. This flexibility allows for some
uncertainties in meeting design remediation criteria, and
allows for possible future treatment process enhance-
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Table 1-1. Examples of Interim (Stabilization) Measures
SWMU
Corrective Measure
Containers
1
2
3
4
5
6
Tanks
1
2
Surface Impoundments
1
2
3
4
5
6
Landfills
1
2
3
4
5
6
7
Waste Plies and Contaminated Soils
1
2
3
4
Overpack Container
Storage Area Construction; Move to New Storage Area
Segregate
Sample and Analyze
Treatment, Storage, and/or Disposal
Temporary Cover
Secondary Containment of Overflow
Leak Detection/Repair, Partial or Complete Removal
Head Reduction,
Free Liquids andj Highly Mobile Wastes Removal
Stabilization/Rep;airof Side Walls, Dikes, or Liner(s)
Temporary Covei"
Runoff/Run-on Cbntrol (Diversion or Collection Devices)
Sample and Analyze for Documentation of the Concentration of Constituents
Left in Place When a Surface Impoundment Handling Characteristic Wastes
Is Clean Closed ;
Interim Ground-Water Measures (see Ground-Water Section of this Table)
Runoff/Run-on Control (Diversion or Collection Devices)
Head Reduction cjn Liner and/or in Leachate Collection System
Leachate Collection/Removal System Inspection or French Drain
Repair Leachate Collection/Removal System or French Drain
Temporary Cap
Waste Removal (see Soils Section of this Table)
Interim Ground-Water Measures (see Ground-Water Section of this Table)
Runoff/Run-on Control (Diversion or Collection Devices)
Temporary Cover
Waste Removal (sfee Soils Section of this Table)
Interim Ground-Water Measures (see Ground-Water Section of this Table)
-------
Table 1-1 (Continued)
SWMU
Soils
1
2
3
Ground Water
1
2
3
4
5
Corrective Measure
Sampling/Analysis, Disposal
Runoff/Run-on Control (Diversion or Collection Devices)
Temporary Cap/Cover
Delineation/Verification of Contamination
Sampling and Analysis
Interceptor Trench/Sump/Subsurface Drain
Pump-and-Treat; In Situ Treatment
Temporary Cap/Cover
Surface Water Releases (Point and Nonpoint)
1
2
3
4
5
Gas Migration Control
1
Particulate Emissions
1
2
3
Other Actions
1
2
3
4
5
6
Overflow/Underflow Dams
Filter Fences
Runoff/Run-on Control (Diversion or Collection Devices)
Regrade/Revegetate
Sample and Analyze Surface Waters and Sediments or Point Source Discharges
Barriers/Collection/Treatment/Monitoring
Truck Wash (Decontamination Unit)
Revegetation
Application of Dust Suppressant
Fencing to Prevent Direct Contact
Sampling Offsite Areas
Alternate Water Supply to Replace Contaminated Drinking Water
Temporary Relocation of Exposed Population
Temporary or Permanent Injunction
Suspend or Revoke Authorization to Operate under Interim Status
Adapted from U.S. EPA, 1989b.
-------
ments (retrofits) in a phased program. For example, de-
signing operational flexibility into a pump-and-treat facility
would allow for a greater range in design flow or would
allow for continued operation if an individual supply well
was not on-line.
At a facility in Indiana, initial sampling showed that
ground water contaminated with VOCs was entering the
facility's storm sewer system and discharging to surface
water. Immediate interim measures taken to prevent mi-
gration off site included storm sewer repairs, installation
of ground-water underdrains beneath replaced storm
sewers, and installation of a carbon adsorption systeijn.
As additional data confirmed source areas and ground-
water flow patterns, more extensive stabilization meas-
ures were taken in progression, including surface watbr
isolation and installation of an extensive ground-water
collection system, air stripping towers, and a carbon ad-
sorption unit to treat the ground water prior to discharge.
Ongoing monitoring activities are being used to deter-
mine whether additional stabilization measures must be
taken and how best to operate these interim facilities.
At a facility in Utah, stabilization measures were also im-
plemented in phases. At this facility, failure of an under-
ground storage tank (UST) automatic shut-off valve
resulted in the release of approximately 27,000 gallons of
jet fuel (JP-4). Immediate stabilization measures included
removing the underground storage tank and collecting
approximately 2,000 gallons of the spilled fuel as free
product. A soil vacuum extraction system was then in-
stalled to remove the contamination remaining in thje
overburden soil. Site monitoring and respiration tests
conducted during the first 10 months of operation showed
some biological activity occurring in the remediation area.
Based on this information and subsequent bioremedia-
tion treatability studies, operating conditions were modi-
fied to promote bioremediation in combination with the
existing vacuum extraction system.
1.6 DOCUMENT ORGANIZATION
This document, organized into four sections, provides the
technical decision guidance for implementing a number
of stabilization technologies. Table 1-2 presents a list of
selected stabilization technologies covered in this hand-
book for containment and treatment of soil and groundt
water media. In most cases, these technologies have
been widely applied to soil and ground-water contamina-
tion. In a few cases, the technology has limited expert
ence in remedial applications, as noted, but has
laboratory or bench-scale experience that appears prom-
ising. In the remaining three sections, selected contain-
ment, soil treatment, and ground-water treatment
technologies, respectively, are discussed. The stabiliza-
tion technology presentations include a description of the!
containment or treatment technology, benefits and limita-^
tions of the technology, critical data requirements, evalu-
Table 1-2. Soil and Ground-Water Stabilization
Selected Containment Technologies
Physical Barriers
Slurry Cutoff Trench/Wall
Sheet Pile Cutoff Wall
Grouting*(1)
Capping
Surface Water Control Methods
Hydraulic Barriers
Drains/Trenches,
Pumping (Extraction/Recharge)(1 )(2)(3>
Gas Venting*(1)(2)
Selected Treatment Technologies
Solidification/Stabilization
Soil Flushing*'1^3'
Bioremediation*(1)(2)(3)
Vacuum Extraction*(1)(2)(3)
Chemical Extraction
Pump-and-Treat*(1)(2)(3)
Notes:
Technology enhancements that may be applicable
to a number of containment and treatment technologies
include:
(1) Horizontal versus vertical well systems
(2) Hydraulic fracturing of contaminated media
(3) Pulsed pumping
ation methods for feasibility and design, and engineering
considerations for implementation.
This document uses, to the extent possible, existing guid-
ance and information developed in various EPA pro-
grams (e.g., Office of Emergency and Remedial
Response, Office of Waste Programs Enforcement, and
Office of Water), as well as state material to assist in per-
forming release characterizations for the various environ-
mental media. As such, many references are provided
which refer the owner or operator to more complete or
detailed information. Where available, identification or or-
dering numbers have been supplied with these citations.
-------
1.7 REFERENCES
U.S. EPA. 1990,a. Assessing UST Corrective Action Tech-
nobgies: Early Screening of Cleanup Technologies for the
Saturated Zone; EPA/600/2-90/027. Risk Reduction Engi-
neering Laboratory.
U.S. EPA. 1990b. Chapter 7, Corrective Action: A Strat-
egy for Protection. In: The Nation's Hazardous Waste
Management Program at a Crossroads: The RCRA Im-
plementation Study. EPA/530-SW-90-069. Office of Solid
Waste and Emergency Response. Washington, D.C.
U.S. EPA. 1990c. The Superfund Innovative Technology
Evaluation Program: Technology Profiles. EPA/540/5-
90/006. Office of Solid Waste and Emergency Response.
Washington, D.C.
U.S. EPA. 1990d. International Evaluation of In Situ Bio-
restoration of Contaminated Soil and Ground Water.
EPA/540/2-90/012. Office of Emergency and Remedial
Response. Washington, D.C.
U.S. EPA. 1990e. Handbook on In Situ Treatment of Haz-
ardous Waste Contaminated Soils. EPA/540/2-90/002. Risk
Reduction Engineering Laboratory. Cincinnati, Ohio.
U.S. EPA. 1989a. RCRA Facility Investigation (RFI)
Guidance - Interim Final. Vols. I - IV, EPA 530/SW-89-
031. Office of Solid Waste, Waste Management Division.
Washington, D.C.
U.S. EPA. 1989b. Corrective Action: Technologies and
Applications. Seminar Publication. EPA/625/4-89/020.
Center for Environmental Research Information.
U.S. EPA. 1988a. RCRA Corrective Action Interim Meas-
ures Guidance. Interim Final. OSWER Directive No.
9902.4. Office of Waste Programs Enforcement. Wash-
ington, D.C.
U.S. EPA. 1988b. Guidance on Remedial Actions for Con-
taminated Ground Water at Superfund Sites. OSWER Di-
rective No. 9283.1-2. Office of Emergency and Remedial
Response. EPA/540/G-88/003 Washington, D.C.
U.S. EPA. 1988c. RCRA Corrective Action Plan - Interim
Final. EPA/530-SW-88-028. OSWER Directive No.
9902.3. Office of Solid Waste and Emergency Response.
Washington, D.C.
U.S. EPA. 1988d. Technology Screening Guide for Treat-
ment of CERCLA Soils and Sludges. EPA/540/2-88/004.
Office of Solid Waste and Emergency Response. Wash-
ington, D.C.
U.S. EPA. 1986a. RCRA Facility Assessment Guidance. NTIS
PB-87-107769. Office of Solid Waste. Washington, D.C.
U.S. EPA. 1986b. Leachate Plume Management.
EPA/540/2-85/004. OSWER Directive No. 9380.0-05.
-------
-------
CHAPTER TWO
Containment Technologies
Containment stabilization technologies can be classified
by media (ground water or soil). In each medium, several
stabilization technologies are commonly used. For ground
water, these technologies include a variety of hydraulic
and physical barriers. The technologies for soil containment
include excavation, capping, and gas venting techniques.
Hydraulic barriers contain contaminated ground water us-
ing hydraulic controls. Containment is achieved by either
(1) recirculating the water within a limited area to prevent
further migration of contamination, or (2) altering the pie-
zometric surface either to decrease the gradient along
which migration is occurring or to divert the contamination
away from a receptor. This technology differs from a
pump-and-treat system in that the pumped or recirculated
water in the containment system is not treated prior to re-
injection, or it may be uncontaminated water recirculated
beyond the boundaries of the plume. The methods of im-
plementing containment through hydraulic control include
pumping or diverting ground water to create a depression
in the piezometric surface, and reinjection or recharge of
water to create a mound or ridge in the piezometric sur-
face. A series of wells is typically used for ground-water
extraction, although trenches or drains may also be used.
Basins, trenches, infiltration galleries, or injection wells
are used for recharging water.
Physical barriers or low permeability vertical barriers can
be used to divert ground-water flow away from a waste
source or to contain ground water contaminated by a
waste source. Three major types of tow permeability verti-
cal barriers are discussed herein: slurry cutoff trench/wall,
steel sheet pile wall, and grout curtain.
The selection of the appropriate containment technolo-
gies is based on an evaluation of each technique in terms
of the use and limitations of the technique, hydrogeologic
conditions, arid contaminant subsurface behavior. The
following sections present a summary of the containment
technologies, their recommended target use and limita-
tions, hydrogeological and chemical data requirements
for evaluation, and evaluation tools for establishing feasi-
bility and a basis for design.
Figure 2-1 summarizes the applicability of the contain-
ment technologies under different site conditions. It also
summarizes the applicability of different evaluation tools
to the technologies.
2.1 DRAINS/TRENCHES
Drains or trenches may be used to collect contaminated
or uncontaminated ground water in order to prevent con-
tamination from reaching downgradient receptors. They
are typically installed perpendicular to the direction of
ground-water flow, although other orientations may be
applicable in certain cases.
Drains are generally constructed by excavating a trench
and installing perforated pipe on gravel bedding on the
bottom of the trench. The trench is then backfilled with
gravel or other envelope material, followed by backfilling
the remainder of the trench with soil. The gravel may be
enveloped in a geotextile fabric to prevent fine soil parti-
cles from entering the gravel and clogging the drain. If
the surrounding soils have a moderately high to high hy-
draulic conductivity and there is some question as to
whether the drain will be a complete barrier, an imper-
meable synthetic membrane may be installed on the
downgradient side of the drain to prevent water from
passing through it.
Interceptor trenches function similarly to drains. Trenches
and drains can be either active (pumped) or passive
(gravity flow). Trenches and drains may be used in the
containment mode for collection of second-phase pollut-
ants that flow on the water table (e.g., light nonaqueous
phase liquids (LNAPLs)). Passive systems are usually
left open with an installed skimming pump or settlement
tank for removal of the pollutant.
The benefits of using interceptor trenches or drains are
(1) they have a relatively simple construction, (2) they are
relatively inexpensive to install, (3) they are useful for col-
lecting contaminants in poorly permeable soils, (4) they are
useful for intercepting landfill seepage and runoff, (5) their
large wetted perimeter allows for high rates of flow, and (6)
it is possible to monitor them to recover pollutants.
The limitations of interceptor trenches or drains are
(1) that open systems require safety precautions to pre-
vent fires and explosions, (2) they are not useful for sites
where contamination is deep, and (3) they may interfere
with other operations at a facility.
2.1.1 Data Collection Requirements
The decision of whether to use trenches or drains de-
pends largely on the depth of the contamination. If the
-------
Site Conditions
High Hyd. Cond.
Low Hyd. Cond.
Shallow WT
DeepWT
Shallow Contamination
Deep Contamination
LNAPL
DNAPL
Confined Aquifer
Unconfined
Complex Stratigraphy
Bedrock
Soils
Evaluation Methods
Numerical Modeling
Analytical Modeling
Water Budget
Flow Analysis
/ ^
X
X
O
0
O
O
I -^
9
O
9
9
9
9
9
9
9
9
9
9
*
\
9
0
9
O
9
O
O
O
9
9
9
9
9
9
9
9
9
/ <*
9
9
9
O
9
0
9
O
9
9
9
O
9
O
9
9
/ V
9
9
9
X
9
X
9
O
9
9
9
X
9
0
9
/ w / o / cr / sjp
ซ
O
*
O
O
O
O
0
O
X
O
X
O
O
X
0
X
ซ
O
X
X
O
O
O
O
X
X
O
O
O
O
O
O
0
O
O
O
O
O
Applicable
O Potentially Applicable
x Not Applicable
Figure 2-1. Applicability of Containment Technologies toiSite Conditions.
water table at a site is relatively shallow, and if the con-
tamination is near the water table either because it (1) is
an LNAPL; (2) is confined to a thin, upper aquifer by un-
derlying strata of low hydraulic conductivity; or (3) is pre-
vented from migrating downward due either to upward
vertical gradients or to having had insufficient time for
vertical migration, then drains or trenches should be con-
sidered as a stabilization technology. Since shallow verti-
cal wells are the main alternative to drains or trenches,
soils with relatively low hydraulic conductivities in whicjh
vertical wells would perform poorly are particularly good
candidates. The areal configuration of the plume may
also create an advantage for drains and trenches; for ex-
ample, a need to create a continuous, linear barrier to
ground-water flow is best met with a drain or trench. The
data requirements that are listed in Table 2-1 are, in most
cases, needed for system design.
2.1.2 Evaluation
The data collected for an evaluation of the feasibility of
using trenches or drains as a stabilization measure is es-
sentially the same as that for any ground-water extraction
10
-------
Table 2-1. Data Requirements for Drains/Trenches
Data Description
Purpose(s)
Source(s)/Method(s)
Depth to aquifer/
water table
Types, thicknesses,
and extents of
saturated and
unsaturated sub-
surface materials
Hydraulic conduct-
ivities and stora-
tivities of sub-
surface materials
Contaminant
concentrations and
areal extent
Piezometric surface
map, ground-water
flow rates, and vertical/
horizontal gradients
Seasonal changes
in ground-water
elevation
NAPL density/
viscosity/solubility
Ground water/surface
water relationship
Locations, screen/
open interval
depths, and pumping
rates of wells
influenced by site
Precipitation/
recharge
Select appropriate
extraction system type
and design trench/drain
Design drainsArenches
Design drainsArenches
Locate and select depth
of drains
Locate and design trenches/
drains
Select depth of
drainsArenches
Predict vertical dis-
tribution of contamina-
tion, design trenches/drains
Design trenches/drains
Determine impacts/
interference
Hydrogeologic maps,
observation wells, boring
logs, piezometers
Hydrogeologic maps, surfi-
cial geology maps/reports,
boring logs, geophysical
surveys
Pumping tests, slug tests,
laboratory permeability
tests
Water quality data
Water level data
Long-term water level
monitoring
Literature
Seepage measurements,
stream gaging
Well inventory, pumping
records
Design trenches/drains
NOAA reports
Note: Table 2-1 applies to hydraulic barrier technology (i.e., well systems).
, 11
-------
system. The evaluation of the data is somewhat different,
however, for different systems. The primary requirements
for trenches or drains are a shallow water table and shbl-
tow contamination. Water level and water quality data
from monitoring wells in the plume are sources of this in-
formation. The direction of ground-water flow must also
be determined so that the location for the drain can be
determined and evaluated.
2.1.3 Engineering Considerations for Implementation
Trenches or drains used in a recirculation system require
excavation of soil along the pathway of the contamina-
tion. This may result in the excavation of a large volume
of contaminated soil which requires special handling and
disposal procedures. In addition, installation of trench lin-
ers and piping may require dewatering of potentially coh-
taminated water. This water will also require special
handling and disposal, unless it can be recirculated. An
alternative would involve the use of a slurry cutoff trench
excavation technique, the installation of a 100 mil thick
high density polyethylene (HOPE) membrane, and the
backfilling of the trench with sand.
Although subsurface drains perform many of the same
functions as pumping well systems, drains may be more
cost effective in certain circumstances. For example,
drains may be particularly well suited to sites with rela-
tively low hydraulic conductivities, where the cost of wells
may be high due to the need to locate wells very close to-
gether. There are, however, a number of limitations to the
use of subsurface drains as a remedial technique. Sub-
surface drains are not feasible at depths exceeding 40
feet due to the difficulty of shoring during installation.
Also, contamination at great depth or in bedrock aquifers
may cause construction costs to be prohibitive, particu-
larly if a substantial amount of rock must be excavateel.
The excavation required for trench or drain installation js
complicated in areas where subsurface utilities and pip-
ing are ubiquitous, or where tanks, buildings, and road-
ways exist. Subsurface drains are also not suitable when
the plume is viscous or reactive because this type of
leachate may clog the drain system. At locations adjacent
to an existing or potential recharge feature, the trench or
drain might require a downgradient impermeable barrier
(U.S. EPA, 1987). Waste compatibility tests are some-
times required to properly select piping and fill material
(U.S. EPA, 1985a,1987).
The design of the trench or drain that is installed as a sta-
bilization measure will probably be based on analytical
solutions to determine discharge rate and anticipated
drawdowns. Lohman (1972) describes an analytical
analysis method. If hydrogeologic conditions are complex
(e.g., drain is to be installed near a significant recharge
source or annual water table fluctuations are severe), a
numerical model may be needed to evaluate the effec-
tiveness of the stabilization action and to alter or expand
the system for the final remedial design.
2.2 VERTICAL WELLS
Vertical wells may be used for extraction or injection of
water, in accordance with the containment system de-
sign. Extraction wells may be either well points, naturally
developed wells, or gravel-packed wells. Well points are
typically joined to a header pipe and pumped with a suc-
tion system. Larger diameter wells are pumped most
commonly with submersible pumps, although vertical tur-
bine pumps may also be used. Extraction wells that are
to be used in a recfrculation system would most likely be
constructed near the leading edge of a plume. Extraction
wells to be used for gradient control would probably be
installed upgradient of the plume.
Injection wells are typically overdesigned in terms of
open area in the screened or open interval because of
the tendency to clog. The overdesign reduces the fre-
quency of maintenance activities such as well develop-
ment and well replacement. Injection wells for a
recirculation system would most likely be constructed
near the origin of the plume of contamination. Injection
wells to be used for plume diversion would be located
outside of the plume along its path of flow.
One of the benefits of vertical wells is that they are a con-
ventional readily available technology. They can be
drilled to essentially any depth, at any location to which
access by the required drilling equipment is available.
Drilling services can usually be obtained on a competitive
bid basis.
The main drawback of vertical wells is the fact that con-
tamination is often spread horizontally more than verti-
cally, so that vertical wells are oriented perpendicular to
the typical Contaminant distribution.
2.2.1 Data Collection Requirements
The data collection requirements for vertical extraction or
injection wells are the same as those listed for drains and
trenches in Table 2-1. When making a decision on
whether or not to use vertical wells in a_stabilization ac-
tion, the primary considerations are the depth and nature
of the contamination and its extent.
2.2.2 Evaluation
The vertical and horizontal extent of contamination, as
determined from the water level and water quality data
measured in monitoring wells, must first be estimated.
The decision of whether or not to use vertical wells can
then be made. If the contamination is too deep to be
reached by trenches or drains, or if property ownership or
land use and utilities preclude their construction, then
vertical wells should be considered. If the contamination
is vertically distributed through the aquifer, then vertical
wells will be an efficient extraction technology. If dense
nonaqueous phase liquids (DNAPLs) have accumulated
deep in the aquifer and are a source of contamination,
vertical wells will be a logical choice to remove or control
them. Reinjection of treated water through vertical wells
12
-------
may be chosen in various situations, such as when aqui-
fer replenishment is needed or when excessive draw-
downs would result from extraction without recharge.
2.2.3 Engineering Considerations for
Implementation
Wellpoint systems are best suited for shallow, unconfined
aquifers where extraction below a depth of 20 feet is not
required. Deep wells and ejector well systems are used
for deeper aquifer systems. Operation and maintenance
costs for pumping systems are high, which may limit their
use for long term containment.
2.3 HORIZONTAL WELLS
The application of horizontal drilling and well installation
technology to enhance containment and treatment stabili-
zation technologies is gaining acceptance by industry. Its
main use in the past has been in the petroleum industry
and in civil engineering applications. The possibility of us-
ing horizontal drilling as a tool for containing or remediat-
ing contaminated soils and ground water is currently
being explored with increasing frequency (Karlsson and
Bitto, 1990). Production rates from horizontal wells are
typically higher than those expected from vertical wells in
the same formation, due largely to the greater screen
lengths possible with horizontal wells (Langseth, 1990).
A custom-equipped drill rig with a slanting rig mast capa-
ble of being oriented from the vertical to 60ฐ-from-vertical
in 15ฐ increments is used in horizontal drilling. The rig and
downhole tools are designed to work as a system, as
shown in Figure 2-2, to drill horizontally on a 100 foot ra-
dius (Karlsson and Bitto, 1990). The drilling assembly is
steerable, and manufacturers indicate that horizontal sec-
tions of screen of greater than 500 feet in length can be
accurately placed at target depths from 18 feet to greater
than 300 feet.
The downhole drilling assembly consists of a dual-wall drill
string and an expanding bit that drills a hole large enough to
permit casing to be installed during drilling. The casing pro-
tects the hole from collapse. Once the well is drilled to the
desired length, an inner drilling assembly is withdrawn and
an HOPE well casing is left in place. The downhole system
is guided using measurements from a tool face indicator
which records the inclination of the drilling assembly and
transmits readings to the surface.
The benefits of horizontal drilling are related to the ability
to install horizontal wells or barriers without the need to
excavate. For example, a series of horizontal wells could
be used to place a pressure curtain of pumped air or
water as a hydraulic barrier or to inject a floor of grout,
epoxy, or cement to create a physical barrier. In applica-
tions such as the recovery of contaminated ground water
beneath structures where vertical wells are less practical,
horizontal wells are a definite advantage (Karlsson and
Bitto, 1990). The use of numerical models (Langseth,
1990) to predict contaminant capture performance indi-
Figure 2-2. Schematic Diagram of Horizontal Drilling Well
System (Eastman Chrlstensen Systems, 1991).
cates that horizontal wells may be advantageous over
vertical wells in certain situations, such as with contami-
nant plumes that are not vertically dispersed.
In addition to pump-and-treat systems and grouting appli-
cations, horizontal wells can be used to facilitate the fol-
lowing treatment-oriented stabilization technologies:
Soil gas vacuum extraction
Steam injection remediation
In situ remediation
Soil flushing
The drawbacks of horizontal drilling are that relatively
specialized equipment is required and it has not yet been
widely used at waste remediation sites. Horizontal drilling
is being used, however, in a soil gas vacuum extraction
system at the Savannah River site in Aiken, South Caro-
lina (Kaback et al., 1991). Two horizontal wells were in-
stalled beneath a leaking pipeline. The lower well will be
used to inject air, while the upper well will be used to ex-
tract the air stream along with volatile organic compounds
(VOCs) that have been stripped from the soil (Karlsson
and Bitto, 1990). Soils in the unsaturated zone and
ground water containing the VOCs will be therefore con-
currently remediated using the paired horizontal wells.
2.3.1 Data Collection Requirements
General data requirements needed to evaluate horizontal
drilling and well installation are the same as those for hy-
draulic barriers, which are listed in Table 2-1. The techni-
cal issues are primarily whether the contamination is
configured such that a horizontal collection system offers
a significant advantage over a vertical one, and whether
trenches, drains, or horizontal wells are more appropriate.
Nontechnical considerations include the availability and
cost of horizontal well construction services at the reme-
diation site. One comparison of horizontal and vertical
well performance in a pump-and-treat system, based on
13
-------
modeling results, indicated that the increased efficiency
of horizontal wells and the resulting savings from a lower
treatment duration can affect the greater capital cost of
well construction (Langseth, 1990). The plume configura-
tion must be well known to correctly position the horizon-
tal well, however, and realize this advantage. The level of
knowledge of contaminant distribution at the stabilization
phase of the site remediation may not be sufficient tp
place a horizontal well optimally.
2.3.2 Evaluation
The technical evaluation of a horizontal well system for
containment or remediation can be performed using ana-
lytical models. Equations that predict the behavior of flow
to horizontal wells are present in the literature (Hantush,
1964; Hantush and Papadopulus, 1962) and were devel-
oped for the petroleum industry. Horizontal well perform-
ance may also be approximated by assuming a series of
closely spaced vertical wells with short screens. Since
the available technical and economic comparison of hori-
zontal wells with vertical wells is not based on actual
sites but on theoretical studies, the evaluation should be
based on access issues and on plume configuration and
contaminant depth.
2.3.3 Engineering Considerations for ;
Implementation
Space requirements for horizontal well bores may require
additional site access considerations. The wellhead is
typically 30 to 60 feet behind the horizontal well screen,
as shown in Rgure 2-2.
The use of geophysical techniques, particularly seismic
methods, has been shown to be helpful in determining
the feasibility of using horizontal wells, determining well
placement, and correlating data. The determination of the
lateral extent of lithplogies in the vicinity of the proposed
horizontal well, particularly aquitards such as clay lenses|,
is essential in designing the specifications of the well.
Geophysical methods and soil borings would be helpful iiji
obtaining this information.
2.4 SLURRY CUTOFF TRENCH/WALL
Slurry walls are often utilized in combination with hydrau-
lic controls or pump-and-treat technologies to iocuk
ground-water recovery on a particular area or to enhance
containment measures. This results in optimal concentra:-
tion of treated water, lower treatment costs, and shorter
cleanup times. Slurry walls are also used with capping
technologies to fully confine a waste area and to prevenjt
clean water from leaching through the wastes.
A slurry cutoff trench/wall is constructed by excavating a
narrow vertical trench, typically 2 to 4 feet wide, and
backfilling with a low-hydraulic conductivity material to
contain a waste source and to prevent contamination
from migrating offsite. The trench, as excavation pro-
ceeds, is filled with a bentonite-water slurry which stabi-
lizes the walls of the trench, thereby preventing collapse;
The slurry penetrates into the permeable soils, creating a
filter cake on the trench walls that seals the soil forma-
tions, prevents slurry loss, and also contributes to the low
permeability of the completed cutoff wall. This narrow
trench is then backfilled with a slurry mixture. Slurry walls
are differentiated by the materials used to backfill the
slurry trench. If a mixture of soil and bentonite is used,
then the wall is known as a soil-bentonite (SB) slurry cut-
off wall. In some cases, the trench is excavated under a
slurry of portland cement, bentonite, and water, and this
mixture is left in the trench to harden into a cement-ben-
tonite (CB) slurry wall. This technique is used at sites
where there is inadequate open area available for the
mixing and placement of the soil-bentonite backfill, where
increased wall strength may be necessary, or where ex-
treme topography changes make it impractical to level-
grade a site.
Slurry walls can be installed in several configurations. Cir-
cumferential installations (totally surrounding the wastes)
are the most common and offer several advantages (U.S.
EPA, 1985b). This placement can reduce the amount of un-
contaminated ground water entering the site, thus reducing
the amount of leachate generated. If pump-and-treat sys-
tems are also used, this placement would reduce the
amount of water to be treated. This placement could en-
hance containment measures if used in conjunction with hy-
draulic controls by helping to maintain an inward gradient
and thus prevent leachate escape. Capping technologies
are often used in conjunction with slurry walls to prevent
leachate generation. An example of a circumferential slurry
wall used in conjunction with capping and pump-and-treat is
the system currently operating at the Gilson Road Super-
fund site in New Hampshire (Weston, 1989). Downgradient
placement of a slurry wall can be used to prevent downgra-
dient migration of contaminated ground water. Examples of
downgradient placement are the systems installed at the
Rocky Mountain Arsenal in Denver, Colorado (CSU, 1988).
A slurry wall could also be installed upgradient of a contami-
nated area to divert clean ground water around a site.
Vertical configurations of slurry cutoff walls may be
"keyed-in" or "hanging." A keyed-in slurry cutoff wall is
excavated into a continuous low-permeability horizontal
confining layer, such as a clay deposit or competent bed-
rock. The confining layer forms the bottom of the con-
tained site. A penetration into the confining layer of 2 to
3 feet is essential for adequate containment. The depth
of the confining layer will also impact the type of excava-
tion equipment used and the completed wall costs. Hang-
ing slurry walls are not tied into a confining layer, but
extend to a hydraulically calculated depth to act as a bar-
rier to the movement of floating contaminants (such as
oil, or fuels) or migrating gases, or for creating an inward
hydraulic gradient used in conjunction with a ground-
water treatment system.
A number of factors may limit the application of slurry cut-
off walls. Site topography can limit the use of a soil-ben-
14
-------
Table 2-2. Soil-Bentonite Permeability Increases Due to Leaching with Various Pollutants
Pollutant
Backfill
Ca** or Mg++ @ 1 ,000 ppm
Ca++ or Mg++ @ 1 0,000 ppm
NH4NO3 @ 1 0,000 ppm
Acid(pH>1)
Strong acid (pH<1)
Base(pH<11)
Strong base (pH>11)
H2S04(1%)
HCI (5%)
NaOH(1%)
Ca(OH)2(1%)
NaOH (5%)
Benzene
Phenol solution
Sea water
Brine(SC=1.2)
Acid mine drainage (FeSO4, pH 3)
Lignin (in Ca** solution)
Organic residues from pesticide manufacture
Alcohol
N
M
M
N
M/H*
N/M
M/H*
N
N
M/H*
M
M
M/H*
N
N
N/M
M
N
N
N
M/H
N - No significant effect; permeability increase by about a factor of 2 or less at steady state.
M - Moderate effect; permeability increase by factor of 2;to 5 at steady state.
H - Permeability increase by factor of 5 to 10.
* - Significant dissolution likely.
+ - Silty or clayey sand, 30 to 40% fines.
Source: D'Appolonia, 1980a; D'Appolonia and Ryan, 1979 (from U.S. EPA, 1984b)
15
-------
tonite wall because the excavation slurry and the backfill
will flow under stress. Thus, the trench line must be
within a few degrees of level. Cement-bentonite waifs
that harden quicker are better suited to irregular topogr^-
phy. If a keyed-in slurry wall is considered, the depth to
and nature of the confining layer becomes a concern.
The layer must have sufficiently low permeability to pre-
vent leakage underneath the wall, it must have adequate
thickness to allow an adequate key (2 to 3 feet), and jit
must be of moderate depth (50 to 70 feet) or excavatioh
of the trench may not feasible.
A major limitation to the application of a slurry cutoff
wall/trench as a physical barrier is the compatibility of th0
cutoff trench backfill mixture with site contaminants. SB
backfills are susceptible to attack by strong acids or
bases, strong salt solutions, and some organic chemi-
cals. Organic chemicals can cause desiccation (drying)
and cracking in soil-bentonite backfill mixtures, resulting
in permeability increases of several orders of magnitude
overtime (U.S. EPA, 1984b). Table 2-2 presents perme-
ability Increases on soil-bentonite due to exposure tb
various contaminants (U.S. EPA, 1984b). Cement-bentor
nite slurry mixtures are more susceptible to chemical at-
tack than most soil-bentonite mixes. i
CB backfills are susceptible to attack by sulfates, strong
acids and bases, and highly ionic substances. In order to
minimize this problem, compatibility testing should be
performed using soil and ground-water samples with the
highest concentrations of contaminants from the site. The
major benton'rte suppliers have developed a bentonit6
that is resistant to chemical attack. The concentration ojf
benton'rte in the mix can be adjusted, or a chemically re^
sistant bentonite or sulfate-resistant cement can be usedj
to provide a satisfactory solution.
Testing parameters to be evaluated in choosing a bento-
nite should include free-swell, filtrate loss, and perme-
ability. The free-swell test would indicate the impact of the
contaminant's effect upon the natural swelling charac-i
teristfes of the bentonite. Filtrate toss tests indicate the deg-
radation of the seal when exposed to the contaminants.
The hydraulic conductivity of a SB wall, with good con-
struction quality control, is approximately 1 x 10"8 cm/sec.
The hydraulic conductivity of a good CB wall is typically
1 x 10"ฐ cm/sec or less (U.S. EPA, 1985b).
2.4.1 Data Collection Requirements
Table 2-3 lists the data needed for design of a slurry cut-
off trench or wall, along with its purpose and potential:
sources.
2.4.2 Evaluation
Because this technology is often implemented in conjunc-
tion with hydraulic controls or pump-and-treat technology,
the evaluation of data generally involves a quantitative;
analysis of ground-water flow and contaminant transport.!
In the initial stages of the decision process, when it must
be determined whether slurry cutoff walls are applicable,
simplified methods such as a water budget analysis, ap-
plications of Darcy's law, and flow net analyses can be
employed. These analyses should focus on flow within
the aquifer and flow underneath the key in the confining
layer. Once the applicability of the technology has been
established, more sophisticated tools such as analytical
or numerical models should be employed to design and
evaluate effectiveness. The combination of cutoff walls
with hydraulic controls usually results in a complex
framework that is difficult to evaluate using simplified
methods. The user is referred to Section 3.1 for a discus-
sion on evaluation techniques.
Other evaluations that must be performed include deter-
mining compatibility of wall construction material with the
existing environment. This involves testing grain size and
permeability of surrounding soils to determine suitability
as backfill materials and determining compatibility of
plume and natural ground-water chemistry with backfill
materials. Laboratory compatibility testing may be re-
quired if published data are not available.
Finally, construction limitations must be evaluated. This
would involve evaluating the amount of area available for
construction site accessibility, irregularities, site topogra-
phy, and potential wetlands issues.
2.4.3 Engineering Considerations for
Implementation
One problem often encountered with keyed-in slurry cut-
off walls is leakage underneath the wall. Leakage has
been observed at the Gilson Road site and the Rocky
Mountain Arsenal (Weston, 1989, and CSU, 1988). This
is often caused by leakage through bedrock fractures ex-
isting beneath the key. The ability to construct an ade-
quate key and maintain overall slurry wall integrity are
key. considerations for implementation. Other considera-
tions for both types of slurry cutoff trench/walls are dis-
cussed below.
Costs for slurry walls are usually expressed in costs per
unit area of wall. Thus, the deeper and longer the trench,
the more costly ft is. Operation and maintenance costs
are negligible. Monitoring of slurry walls usually involves
monitoring ground-water levels inside and outside the
wall to ensure that design head levels are not exceeded.
Ground-water quality monitoring is used to determine the
effectiveness of the entire remedial effort.
2.4.3.1 Soil-Bentonite Slurry Cutoff Trench/Wall. A
SB cutoff trench/wall requires a relatively flat area for
construction with a parallel earth berm constructed to
confine the movement of slurry from the mixing operation
away from the construction area. A typical SB slurry cutoff
trench/wall installation requires space.for bentonite storage,
Slurry preparation equipment, water storage tanks,
hydradation pond, circulating pumps and slurry storage
pond, de-sanding area (if required), a trench spoils area
where excavated soils are placed adjacent to the trench,
16
-------
Table 2-3. Data Requirements for Slurry Cutoff Trench/Wall
Data Description
Purpose(s)
Source(s)/Method(s)
Site accessibility
Topography
Depth to continuous
impermeable strata
or competent bedrock
Heterogeneity of
subsurface
formation
Vertical and
horizontal hydraulic
conductivity of
confining layer
Excavated soil type
Degree of bedrock
fracturing
Ground-water depth,
rate and direction
of flow
Hydraulic
conductivity of
contaminated soil
Soil chemistry
Chemistry of waste
and ground water
Select wall type
Soil-bentonite walls
require large land
area with relatively
flat topography
Selection of keyed-in
or hanging wall
Selection of wall type;
excavated material
may not be appropriate
to mix
Determine suitability
of layer as a key
Suitability for use as
trench backfill material
Evaluate potential for
contaminants to migrate
underneath the key
Establish potential for
installation of hanging
wail with design of
inward gradient for pump-
and-treat scheme
Evaluate effectiveness
of slurry wall pump-and-
treat systems
Cement and bentonite can
be modified to
accommodate chemistry
Compatibility testing of
cement or bentonite and
wall material with contam-
inated ground water and
soil
Site inspection
USGS topography map,
site inspection, site-specific
topographic/contour maps,
water level maps
Borings, geophysical survey,
bedrock and surficial
geology maps
Surficial geology maps,
test pits, soil borings,
geophysical survey
Slug tests, laboratory tests
Gradation analyses,
permeability tests
Rock cores, boring logs,
geology maps
Existing hydrogeologic
maps, boring logs,
observation wells,
piezometers
Pumping tests, slug tests
Soil sampling
and analysis
Ground-water sampling and
chemical analysis,
filtrate loss, free
swell, and permeability
testing
17
-------
and a backfill mixing area where bentonite is mixed with
the backffll soils. A typical slurry wall construction site is
shown in Figure 2-3. Figure 2-4 shows a cross section of
a soil-bentonite slurry cutoff trench, including excavatioh
and backfill operations.
The type of trench excavation equipment used depends
upon the depth and length of the cutoff wall. For soil-ben-
tonite walls, a backhoe is typically used to depths of up to
50 feet. Crane-operated clamshells attached to a kelly
bar can excavate to depths of up to 200 feet. Percussion
tools or chisels are used when boulders, cobbles, ancJ
other hard consolidated materials are encountered which
cannot be removed with standard equipment or when
"keyfng-in" of the trench into bedrock is required.
Field quality control during installation is essential during
the installation of a slurry cutoff trench/wall.
2.4.3.2 Cement-Bentontte Slurry Cutoff Trench/Wall.The
principal difference between CB slurry cutoff
trenches/walls and SB slurry cutoff trenches/walls is the
backfill. This main difference produces differences in
applications, compatibility, and costs. The initial
excavation process for the CB wall is conducted in the
same manner as the SB wall; however, the excavated
soils from the trench are not reused to form the wall!
Instead, the material is removed to a disposal areai
Backfilling Js performed using a cement-bentonite water
mixture with set retarders, which are added so that the;
wall will slowly set. I
The trench for a CB cutoff wall can be excavated with a
clamshell bucket. This method involves the excavation of a
series of primary panels 6 to 10 feet in length. Once the prn
mary panels have set, secondary panels are excavated
through the gaps in the primary panels and keyed intoj
them. The cement-bentonite of the secondary panels bonds
with the primary panels to create a tight cutoff. As with a SB!
wal, field quality control (QC) during testing is essential. '
2.5 SHEET PILE CUTOFF WALL
Sheet pile cutoff walls can be used to contain contami-
nated ground water or to divert ground-water flow around'
or below contaminated areas. Sheet piles can be inter-;
locking steel, precast concrete, or wood sections. The'
use of wood or precast concrete sections as a ground-;
water barrier is not appropriate for interim stabilization;
because wood is an ineffective water barrier, and precast
concrete is used only in a situation where great lateral re-
sistance is required. Sheet pile cutoff walls are con-
structed by driving individual sections of interlocking steel j
sheets into the ground using single, double-acting impact i
or vibratory pile drivers to form a thin impermeable barrier
to ground-water flow. Initially, this type of wall is porous at
the section interlocks, but will fill with fine sand and sift. If
there are little or no fines present in the soil, tremie grouting
along the interlocks can be used to create a barrier.
Backfilled Backfill Area of Active Proposed Line
Trench Placement Excavation of Excavation
Area
Access
Road
Slurry . ,
Preparation) I
Equipment I_1
000
Water Tanks
Figure 2-3. Typical Slurry Wall Construction Site (U.S.
EPA,1985b).
Figure 2-4. Soil-Bentonite Slurry Cutoff Trench Cross Sec-
tion (U.S. EPA, 1985b).
Sheet piling is typically a less permanent measure than
slurry walls and may be installed quicker with less effort
and costs. Because of unpredictable wall integrity, sheet
piling is seldom used in hazardous waste applications ex-
cept for temporary dewatering for construction or excava-
tion purposes. However, sheet piling could be effective
as a short-term immediate measure to enhance recovery
or containment. One of the largest drawbacks of sheet
piling is that it is difficult to install in rocky soils. Damage
to or deflection of the piles is likely to render a sheet pile
ineffective as a ground-water barrier. Furthermore, it is
-------
difficult to use sheet piling for deep ground-water prob-
lems due to the pile driving limitations. Sheet piles can be
driven into bedrock, but the adequacy of the key formed
would be questionable. Grouting could be employed to
seal the bottom of the pile; however, applications have
not been documented.
2.5.1 Data Collection Requirements
Table 2-4 lists the data required, the purpose, and possi-
ble sources of information for designing a sheet pile cut-
off wall.
2.5.2 Evaluation
The evaluation of this technology would involve determin-
ing if the pile could be driven to the desired depth. If cob-
bles or boulders exist or if the ground-water problem is
deep, this technology may not be feasible. It must also be
determined whether the type of piling is compatible with
the surrounding environment. If the environment is too
corrosive, then long-term application of this technology
should be avoided.
2.5.3 Engineering Considerations for
Implementation
Soil type and waste characteristics are important factors
to consider in the use of steel sheet pile walls as a physi-
cal barrier, because there is a high potential for leakage
through interlocking piles or if individual sheets encounter
boulders. Sheet piles are typically used in loose to me-
dium dense granular soils that predominantly consist of
sand and gravel. To serve as an effective cutoff, sheet
pile walls should extend to bedrock or into a low-perme-
ability soil strata. The maximum depth to which sheet
piles can be driven without damage to the interlocks be-
tween individual sections is typically about 40 feet. The
characteristics of the waste constituents strongly affect
the expected service life of the sheet-pile wall (particu-
larly the pH and conductivity of the waste material).
Sheet piles are not suitable for very dense soils or soils
with boulders present because the sheet piles will be
damaged during installation. They are also not suitable
for ground water containing high concentrations of salts
or acids unless they can be coated with a coal tar epoxy
or cathodic protection can be used.
2.6 GROUTING
Grouting is a process by which a fluid material is pres-
sure injected into soil or rock to reduce fluid movement
and/or impart increased strength. Grouts accomplish this
through their ability to permeate voids and gel or set in
place. Grouting can be used to control the movement of
ground water and to solidify or stabilize a soil mass.
Grouts injected into a soil mass reduce the permeability
of the deposit. Grout curtains can be created in uncon-
solidated materials by pressure injection.
Grout types are divided into two general classifications
participate or suspension grout and chemical grout. Par-
ticulate grouts are fluids that consist of a suspension of
solid material such as cement, clay, bentonite, or a com1
bination of these materials. These materials are usually
the more viscous of the available grouting materials and
have the largest particle size. Chemical grouts are fre-
quently classified into two major groups: silica or alumi-
num based solutions and polymers. Chemical grouts rely
on polymerization reactions to form hardened gels. They
have initially low viscosities and can therefore be used in
finer grained soils. Types of grouts include portland ce-
ment grouts, cement-bentonrte grouts, silicate grouts,
and organic polymer grouts.
Because of costs, grouted barriers are seldom used for
containing ground-water flow in unconsolidated materials
around hazardous waste sites. Slurry walls are less
costly and have lower permeability than grouted barriers.
Consequently, for waste site remediation, grouting is best
suited for sealing voids in rocks. The compatibility of
grouts with hazardous waste has not been thoroughly
studied, and only general incompatibilities are known.
However, some data suggest that organic chemicals can
be detrimental to several types of grout.
The chemical compatibilities of various types of grouts
are presented in Tables 2-5 and 2-6.
2.6.1 Data Collection Requirements
Table 2-7 is a list of data needed to design a grouting
program. It also lists the purposes for the data and possi-
ble sources.
Field investigations for a grouting scheme for overburden
soils involve (1) obtaining an accurate definition of the
soil profile (i.e., depth and extent of soil strata), and
(2) obtaining specific information concerning soil proper-
ties that control groutability (i.e., field permeability tests,
porosity or void ratio, and laboratory permeability tests).
Field permeability can be determined from falling-head or
rising-head permeability tests or pumping tests.
For bedrock grouting, key information is (1) the recording
of water loss during drilling, and (2) rock core logging
which describes in detail jointing, fracturing, weathering,
and Rock Quality Designation (RQD).
2.6.2 Evaluation
Soil laboratory testing would be performed to determine
grain size and permeability. The soil samples should be
reclassified to ensure that proper stratigraphy of the site
is identified. The stratigraphy is used to determine the ap-
propriate grouting methods and procedures.
Grain size analysis is used to determine whether the de-
posit can be grouted. Although soils with greater than
10 percent by weight passing a No. 200 sieve can be
grouted, it is generally very expensive to do so. This is
due to the fact that chemical grouts are more expensive
per gallon of solution than paniculate grouts.
The geochemistry of a site, including that caused by con-
taminants from waste disposal, is extremely important to
19
-------
Table 2-4. Data Requirements for Sheet Pile Cutoff Wall
Data Description
Purpose(s)
Source(s)/Method(s)
Subsurface
soil conditions
Depth to bedrock
(Impermeable strata)
Grain size
distribution
In-situ soil
density
Depth to ground-water
table
pH of ground water
and waste
Determine feasibility. If
boulders or vefy dense
soils are present, very
difficult to install wall
for cutoff purposes
Optimal wall depth
Determine potential for
fine grained soil particles
to fill interlocks between
sheet pile sections
Penetration resistance
affects depth i
Maximum depth to
which steel sheet piles
can be effectively
driven is approxi-
mately 40 to 50 feet
Determine if corrosive
setting, establish need
for coating piles or
using cathodic protection
Surficial geology
maps, test pits,
borings
Geologic maps,
borehole logs
Sieve analysis
Standard penetration
test (SPT) Results from
borings
Hydrogeologic maps,
observation wells,
data boring logs,
geophysical
survey
pH analysis
and conductivity
testing during
borehole drilling
a grouting program. Among the geochemical data that
should be determined during a design phase are the na-
ture and extent of waste/contaminants at the site and the
presence of soil or rock layers (such as salt deposits) that
may impact grout solution chemistry.
2.6.3 Engineering Considerations for ;
Implementation
A key consideration for implementation of the technology
is the compatibility of the grout with the wastes. In theory,!
grouting could control contamination migration. However,;
in many cases, the waste/grout interaction and compati-
bility cannot be predicted and extensive testing is re-
quired (U.S. EPA, 1984b). j
Grouting is typically a specialty operation. It is performed
by a limited number of contractors, and each program is
highly site specific. Explanations of different grouting ap-
plications are provided in the following section.
The selection of a grout type must include an evaluation of
required soil permeability as well as grout, gel time, setting
characteristics, volume of grout, and penetration. The appli-
cability of different classes of grouts, based on soil grain
size, is presented in Figure 2-5. Layout of grout injection
pipes depends upon soil types, grout viscosity, injection
pressure, and gel time. Spacing will depend upon grout
penetration and desired grouted soil properties.
Grout curtains are formed by pressure injecting the grout-
ing material through a pipe into the strata to be sealed.
The injection points are typically arranged in a triple line
of primary and secondary grout holes. A predetermined
amount of grout is pumped into the primary holes. After
the grout in the primary holes has had time to set or gel,
secondary holes are injected. The secondary grout holes
are intended to fill in any gaps left by the primary grout in-
jection. A typical grout pattern is shown in Figure 2-6. Pri-
mary holes are typically spaced at intervals of 3 to 5 feet.
J20
-------
Table 2-5. Interactions between Grouts and Generic Chemical Classes
Grout Type
Chemical Group
Portland Cement
Type I Type II and V
Bentonite Cement-Bentonite Silicate Acrylamide
Acid
Base
Heavy Metals
Nonpolar Solvent
Polar Solvent
Inorganic Salts
1d
1a
2c
2d
2c
2c
1a
1a+
2a
2d
2?
2a
?c
?c
?d
?d
?d
2d
?c
?d
2c
?
?
?d*
3a
2c
3?
?
?
3?
2c
3d
2?
?a
?a
3d**
KEY: Compatibility Index
Effect on Site Time
1 No significant effect.
2 Increase in set time (lengthen or prevent from setting).
3 Decrease in set time.
Effect on Durability
a No significant effect.
b Increase durability.
c Decrease durability (destructive action begins within a short time period).
d Decrease durability (destructive action occurs over a long time period).
* Except sulfates, which are ?c.
+ Except KOH and NaOH, which are 1d.
> Modified bentonite is d.
? Data unavailable.
** Except heavy metals, which are 2.
Source: U.S. EPA, 1984b.
| Ml Grout
Acrytamkln
SfcjlM 1
| Bertonle | ]
| Cement |
ซ" ' 10 t.O 0.1
Grain Size kiMHrmin
Iriii, J Gravel Sand
"""^ Comป | Fine Con* | Modun | Fine
....J
a
n
-Secondary Grout Column
Primary Grout Pipe -
r____?
BaaicCed
L- Secondary Grout Pipe
Primary Grout Cokimn
Figure 2-5. Soil Gradation versus Grout Type (Adapted
fromU.S. EPA, 1985).
Figure 2-6. Typical Grout Pipe Layout for Grout Curtain
(U.S. EPA, 1984a).
21
-------
Table 2-6. Interactions between Grouts and Specific Chemical Classes
Grout Type
Chemical Group
Portland Cement
Type I Type II and V
Bentonite Cement-Bentonite Silicate Acrylamide
Organic Compounds
Alcohols and Glycols
Aldehydes and Ketones
Aliphatic and Aromatic
Hydrocarbons
Amides and Amines
Chlorinated Hydrocarbons
Ethers and Epoxides
Heterocyclics
Nitrites
Organic Acids and Acid
Chlorides
Organometallics
Phenols
Organic Esters
?d
?
2a
?
2d
9
?
?
1d
?
1d
9
?d
?
2?
?
2d
?
?
?
1d
?
?
?
?d
?d
?d
?d
?d
?d
2 Increase in set time (lengthen or prevent from setting).
3 Decrease in set time.
Effect on Durability ;
a No significant effect. i
b Increase durability.
c Decrease durability (destructive action begins within a short time period).
, d. Decrease durability (destructive action occurs over a long time period).
* Except sulfates, which are ?c. I
+ Except KOH and NaOH, which are 1 d. i
> Modified bentonlte is d. !
I
? Data unavailable. '
?d
?a
?d
?a
?a
?a
?a
?a
2a
?a
1a
Inorganic Compounds i
Heavy Metals, Salts, and 2c 2a
?d 2c ?a 2?
Complexes i
inorganic Acids 1d 1a
Inorganic
Inorganic
Bases 1a 1a+
Salts 2d 2a
KEY: Compatibility Index
Effect on
1 N<
Site Time
3 significant effect.
?c> ?c ? 2c
?c> ?d ? 3d
2d ?d* 1a 3d
Source: U.S. EPA, 1984b.
22
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Table 2-7. Data Requirements for Grouting
Data Description
Purpose(s)
Source(s)/Method(s)
Soil permeability
Determine grout types
applicable for subsurface
conditions
Laboratory permeameter
testing on soil samples;
falling head field permeability
testing in borings
Grain size
distribution
Determine if soils are
within range of soils
that can be grouted
and type of grout
applicable
Sieve analysis
Chemistry of soil,
waste, and
ground water
Compatibility with grout
determination of design
mix
Ground-water sampling
and chemical analysis
Heterogeneous nature
of surficial geology
Determine type of grout
and installation
technique, i.e., reduce
pressure for cement
grout injection; adjust
set time for chemical grout
Surficial geology maps,
test pits, borehole
soil sampling
Depth to and type
of bedrock
Determine depth of wall
and need for grouting of
fractured zones to
competent bedrock
Bedrock geology maps,
geophysical surveys,
bedrock outcrop
mapping, borehole
logs, bedrock coring
Depth to ground-
water table
Determine depth
pressure grouting to
stop
Existing hydrogeologic
maps, observation wells
and piezometer records,
borehole logs
Direction and rate
of ground-water,flow
High ground-water flow
adversely affects
certain integrity
Regional hydrogeologic
maps/reports, water level
records in borings,
monitoring wells
Ground-water pH,
sulfides, and calcium
Integrity of grout curtain
Sampling and analysis
23
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Grout curtains are keyed into impermeable soil strata 6r
competent bedrock. Grout curtains, like the other physi-
cal barriers, are applied to a site in various horizontal
configurations, the most common being circumferential
and downgradient.
Area grouting is a low pressure technique used to form a
grout blanket. This is accomplished by injecting grout into
a series of closely spaced injection holes on a grid pat-
tern. Area grouting may be appropriate to in situ waste
immobilization by injecting into the wastes, depending
upon the compatibility of the wastes with the grouts.
In situations where a waste has migrated beneath buildings
or other facilities, horizontal drilling can be used for injecting
grout under pressure to seal/immobilize the contaminants.:
2.7 CAPPING
Capping or surface sealing is part of a closure process in
which a buried waste or contaminant plume is isolated to
avoid surface water infiltration, thereby minimizing the gen-
eration of teachate. Capping may also be used to control
the emission of gases and odors, reduce erosion, and
improve aesthetics. Capping provides a stable surface
which prevents direct contact with wastes, and is neces-
sary when contaminated materials are to be buried or left
in place at a site. In situations where the waste is entire|y
above the zone of ground-water saturation, a properly de-
signed cover can prevent the entry of water into trie
closed landfill or into a surface impoundment that has
been closed as a landfill. In general, capping is performed
when extensive subsurface contamination at a site pre-
cludes excavation and removal of wastes due to potential
hazards and/or unrealistic costs. Capping is often pe|r-
formed in connection with ground-water extraction or contain-
ment technologies (i.e., physical barriers or hydraulic barriers).
2.7.1 Data Collection Requirements
General data requirements for capping are summarized
in Table 2-8. \
2.7.2 Evaluation
Surface water runoff rates for landfills, surface impound-
ments cbsed as landfills, or other areal sources can be esti-
mated with several methods. Fenn et al. (1975) discuss the
"water balance method." Given monthly values for precipi-
tation and potential evapotranspiration, estimates of
monthly evapotranspiration, runoff, and infiltration can b;e
obtained for different types of soils. Seepage rates through
multilayered soil columns can be estimated through succes-
sive appKcatfons of the method. Thus, the water balance
method is applicable to an areal source with a cap. Dass et
al. (1977) reported on a similar method.
A somewhat more sophisticated method is incorporated
In the Hydrologic Evaluation of Landfill Performancjs
(HELP) model (U.S. EPA, 1984c). The HELP program (s
an easy-to-use model that was developed to assist land-
fill designers and regulators by providing a tool to alloW
rapid, economical screening of alternative designs. HELP
is a quasi-two-dimensional model that computes a daily
water budget for a landfill represented as a series of hori-
zontal layers. Each layer corresponds to a given element
of a landfill design (e.g., cap, waste cell, leachate collec-
tion system, and liner). HELP considers a broad range of
hydrologic processes including surface storage, runoff,
infiltration, percolation, evapotranspiration, lateral drain-
age, and soil moisture storage. The HELP model requires
climatotogic data, soil characteristics, and design specifica-
tions as inputs. Climatobgic data consist of daily precipita-
tion, mean monthly temperatures, mean monthly solar
radiation, leaf area indices, root zone or evaporative
zone depths, and winter cover factors. Soil charac-
teristics include porosity, field capacity, wilting point, hy-
draulic conductivity, water transmissivity, evaporation
coefficient and Soil Conservation Service (SCS) runoff
curve numbers. Design specifications consist of the number
of layers and their type, thickness, sbpe, and maximum lat-
eral distance to a drain, if applicable, or whether synthetic
membranes are to be used in the cover and/or liner.
While the water balance method can be solved by hand,
the large number of calculations performed by HELP are
most efficiently done on a computer. The program is op-
erational on EPA's National Computer Center in Re-
search Triangle Park, North Carolina (U.S. EPA, 1985b).
Bicknell (1984) has modified HELP to simulate chemical
losses from a landfill. Both leaching and volatilization
losses can be estimated. HELP has been used in the
analysis of existing landfills and the design of new sites.
The results of HELP may be used to compare leachate-
production potential of alternative designs, select and
size appropriate drainage and collection systems, and
size leachate-treatment facilities. The HELP model is
available on diskette from the EPA Office of Research
and Development in Cincinnati, Ohio.
2.7.3 Engineering Considerations for Implementation
The final cover minimum thicknesses recommended by
EPA for a multilayered cap (U.S. EPA, 1989) from final
grade are as follows:
Vegetative and protective layerA 24-inch thick layer
of topsoil or soil fill
Drainage layer12 inches of sand (permeability
1 x 10"2 cm/sec)
First barrier layer componentSynthetic membrane
(20 mil thickness minimum)
Second barrier layer component24 inches of low
permeability compacted soil with a maximum in-place
permeability of 1 x 10"7 cm/sec
Gas vent layer (optional based upon site-specific con-
ditions)12 inches of native soil or sand to act as a
foundation for the cap or to vent/control gas
Waste
; 24
-------
Table 2-8. Data Requirements for Capping
Data Description
Purpose(s)
Source(s)/Method(s)
Extent of contamination
Depth to ground-water
table
Availability of cover/
capping materials
Soil characteristics
- Gradation
- Permeability
(percent
compaction,
moisture
content)
- Strength
Climate (precipitation)
Final land use
Determine cost
effectiveness of cap
vs. excavation/removal
May not be effective in
areas with a high
ground-water table
Implementability
and cost
Suitability for:
- drainage layers
- impermeable soil
layer
- mixing with bentonite
Slope stability
Expected infiltration
rate; design criteria
Selection of proper
cap design
Surficial soil and bore-
hole sampling and
analysis to determine depth and lateral
extent of contamination
Hydrogeologic maps,
observation wells, and
borehole logs
Local borrow pits/quarries,
surficial geology maps
Laboratory testing of soil
samples
- Sieve analysis,
Atterberg Limits
- Moisture/density
relationships,
permeability testing
in triaxial cell per
Army Corps of Engineers
procedure
- Triaxial shear, direct
shear testing
NOAA records;
local rainfall records
The final design profile of a typical multilayered or RCRA
cap will also include geotextiles as a filter between the
vegetative/protective layer and drainage layer and/or a
protective layer over the synthetic membrane.
These performance standards may not always be appro-
priate, especially in instances where a cap is intended to
be temporary, where there are very low amounts of an-
nual precipitation, or when the capped waste is immo-
bile. In such cases, the layer thickness of the cap can be
reduced, and only one low permeability layer may be
necessary. Alternate designs must provide long-term
performance at least equivalent to the recommended
RCRA design. All alternative designs must be approved
by EPA.
The selection of capping materials and a cap design are
influenced by site-specific factors such as local availabil-
ity and costs of cover materials, desired function of the
cover, the nature of the wastes being covered, local cli-
mate (rainfall), site topography, hydrogeology, and pro-
jected future use of the site.
25
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The most commonly used materials for the impermeable
or barrier layer are low-permeability soils such as silty
clay and synthetic membranes. The low-permeability
soils are typically fine-grained natural soils (silts and/or
clay) that can achieve a minimum in-place compacted
permeability of 1 x 10"7 cm/sec. '
At sites where these soils are not readily available, ad-
mixtures to granular soils have been utilized to achieve
the required permeability. Recently, bentonite mixed with
granular soils has been accepted as an alternative to-
day by state agencies for use as bottom containment
systems and caps for municipal solid waste landfills.
A soil-bentonite cap is a barrier composed of granular
soil and bentonite. When contacted with water, bentonite
swells and fills the voids in the soil, thus preventing thfe
passage of water and other liquids. The two principal ad-
vantages of a bentonite cap are the low permeability im-
parted to readily available granular soils by the addition
of the bentonite and the self-healing nature of the cap.
The base material in the bentonite is a high-swelling soj-
dium montmorillonite that has a unique molecular struc-
ture enabling the clay to absorb many times its owr|i
weight in water. Good quality sodium bentonite will swell
10 to 15 times the dry bulk volume. The molecular struc-
ture of bentonite accounts for its ability to swell to a
much greater volume than other natural clays.
Typically, the higher the permeability of the soil to be
sealed, the larger the quantity of bentonite required. For
hazardous waste sites, soil, bentonite, and water should
be fully blended in a computer-controlled pugmill. Soil is
dropped onto a conveyor belt and is continuously
weighed Bentonite is then added to the soil at the specif
fied rate. The bentonite and soil are mixed in a pugmill
with a metered amount of water, bringing the mixture to
'the optimum moisture content. After mixing, the soil-ben!-
tonite is transferred to the area for application by an asL
phalt paving machine which precisely distributes the
blended soil-bentonite mixture. The mixture is then com;
pacted to the specified density using smooth steel drum
vibratory roller/compactors to achieve the required in--
place permeability. Quality control testing requirements
for the installation of soil-bentonite caps are presented in
Table 2-9.
The advantage to using a soil-bentonite admixture is the
high degree of quality control and compaction that cari
be achieved. When properly mixed, the soil-bentonite will
have the hydraulic characteristics of a clay but the physi-r
cal characteristics of the granular soil with which the benT
tonite was mixed.
Rexible synthetic membranes typically used for capping
operations are polyvinyl chloride (PVC), HOPE, very low
density polyethylene (VLDPE), and Hypalon. Synthetic
membranes range in thickness from 20 mil to over
100 mil, and are available in sheets of varying width and
length. Adjacent sheets are overlain in the field and
bonded or welded together depending upon manufactur-
ers' specifications. The chemical resistance of a syn-
thetic membrane used for a cap is usually not critical.
The thickness and flexibility of a synthetic membrane
should be carefully evaluated during material selection.
The synthetic membrane should be of adequate thick-
ness to prevent failure under potential stresses during
the postclosure care period or from construction. The
adequacy of the selected thickness should be demon-
strated by an evaluation of the type, strength ,and dura-
bility of the membrane; and of site-specific factors such
as steepness of slopes, physical compatibility with the
material used in the underlying and overlying layers,
stresses of installation, expected overburden, climate
conditions, and settlement or subsidence. Field seaming
of the membrane must be performed by qualified and ex-
perienced installers.
The design of the bw permeability compacted soil layer will
depend upon site-specific factors including the physical
properties and engineering characteristics of the soil being
compacted, the degree of compaction obtainable, the ex-
pected loadings, and expected precipitation. A minimum
24-inch thick layer is recommended in RCRA guidance
(U.S. EPA, 1989). This thickness is based upon constructa-
bility considerations and the ability to provide uniform per-
meability for the natural soils of the entire area. For
example, silty clay deposits can have fine sand lenses,
which can result in localized areas of in-place compacted
permeabilities of greater than 1 x 10"7 cm/sec.
Recently, bentonite panels consisting of a dry granular
sodium-bentonite layer approximately 1/4-inch thick with
a woven polypropylene geotextile on each side have
been used to cap an NPL site in New York State (Hud-
son River PCB Remnant Sites). These panels have the
same self-healing and self-sealing characteristics of soil-
bentonite. The advantages of this method are flexibility
allows rapid and easy installation; a small crew can eas-
ily perform the installation; all seams are simple overlap
(6-inch) seals; panel can be cut or trimmed with a utility
knife; permeability is less than 1 x 10'7 cm/sec (can
achieve 1 x 10"9 cm/sec); material is uniform, contained,
and rehydrates quickly; subgrade bearing capacity is not
a factor; and less truck traffic is required.
The following are key design considerations for a cap (a
detailed discussion is provided in U.S. EPA, 1989):
The slope of the low-permeability layer should be be-
tween 3 and 5 percent to prevent erosion and ponding
of rain water on the top of the cap. The perimeter side
slopes are final grades and should be no steeper than
three (horizontal) to one (vertical). For each 20-foot in-
crease in vertical heights, a bench should be con-
structed in the slope to control surface water runoff
and subsequent erosion (U.S. EPA, 1985b).
The impermeable barrier portion of the cap should be
located beneath the average depth of frost penetration
26
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Table 2-9 Quality Control Requirements for Soil-Bentonite Caps
Item
Source(s)/Method(s)
Frequency
Develop bentonite
application rate to
soil to achieve
required permeability
for specified compaction
Grain size distribution
Moisture density
relationship
Permeability testing
Field construction quality control
Moisture density
relationship
ASTMD 422,0 1140
ASTMD1557
Triaxial method
with back pres-
sure measurements
per U.S. Army
Corps of Engineers
procedures
EM-1110-2-1906
ASTMD 1557
Minimum 3 tests per source
Minimum 3 tests per source
Minimum 3 tests per source
One 5-day shift
In-place density
ASTMD 1556
Sample on grid, spacing of
50 feet
In-place moisture
content
Grain size distribution
Bentonite content test
ASTMD 2216
ASTM D 422,
D1140
ASTMD 1140
Sample on grid, spacing of
50 feet
One per 300 cy placed
measured at mixing plant
One per 200 cy placed'
measured at mixing plant prior to
mixing with water
for the site, as determined by the U.S. Department of
Agriculture mapping.
The vegetative layer should be thick enough to contain
the effective root depth or irrigation depth for the type
of vegetation planted.
The drainage layer should be designed and con-
structed to discharge flow freely in the lateral direction
to exit the cap.
Several materials and designs are available for capping.
Factors influencing the proper selection of materials and
design include desired functions of cover materials,
waste characteristics, climate, hydrogeology, projected
land use, and availability and costs of cover materials.
More detailed sources of information concerning design
considerations for specific types of caps are Lutton
(1979), U.S. EPA (1985b), U.S. EPA (1985a) and U.S.
EPA (1989).
27
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Surface seals require long-term maintenance. Periodic
inspections should be made for settlement, ponding of
liquids, erosion, and invasion of deep-rooted vegetation.
Concrete barriers and bituminous membranes are vul-
nerable to cracking, but the cracks can be relatively eas-
ily repaired (U.S. EPA, 1987).
2.8 SURFACE WATER CONTROL METHODS I
Surface water controls are required in instances where
runoff from a site requiring remedial actions may trans-
port contaminants from the previously contaminated
area. There are several different methods for the control
of run-on and runoff. These methods include grading and
various forms of surface water diversion. A number of
methods may be used in combination to obtain the ap-
propriate degree of surface water control.
Grading describes the reshaping of the surface of a par^
ticulars'rte in order to reduce infiltration and erosion while
redirecting runoff from the site. This is accomplished
through the use of earth-moving machinery and the ad-
dition of fill where necessary. The benefits of grading
include:
Relatively low cost if cover materials are readily
available
Reduction of ponding so as to minimize infiltration |
Reduction of runoff velocity and soil erosion ;
Limitations of grading are:
It is not applicable to smaller areas of land where
there is no room for gradual grading
Costs increase significantly if appropriate fill is not
available within a reasonable distance
Dikes and berms are one method of surface water diver-
sion used for surface areas no greater than 5 acres.1
These can be used to divert surface water from a site or
to prevent mixing of different types of wastes. A dike or
berm is constructed by the addition of several lifts of soil,
each being adequately compacted, until the desired sizei
is obtained. These structures are constructed around the!
contaminated area so as to redirect surface flow to drain-;
age ways and away from the contamination.
Benefits of dikes and berms include: !
Construction costs are low assuming that the required
soil is readily available :
They can be used to provide protection against site
flooding I
They can be constructed quickly with readily available
contractors and equipment !
Limitations of dikes and berms include: !
Structures are temporary and are usually not used for
a period greater than 1 year
If improperly designed, constructed, or maintained,
dikes or berms may increase the infiltration into the
ground water
An additional method of surface water diversion is the
use of channels and waterways. Channels are excavated
ditches that can be used to intercept runoff. Waterways
are usually either sodded or lined with rip-rap and are
used to carry larger amounts of flow (usually from areas
larger than 5 acres), Construction of channels and water-
ways is simply a matter of excavating to the desired di-
mensions and lining the channel or waterway
appropriately.
Benefits of channels and waterways include:
Simple, well-established construction techniques mini-
mize construction costs and use readily available con-
tractors and equipment
They are more appropriate for the collection and
transfer of water than berms and dikes.
They are more permanent than berms or dikes.
Limitations of channels and waterways include:
Maintenance is required, especially in sodded water-
ways
Improperly constructed, designed, and maintained
channels and waterways can increase erosion and the
rate of surface water infiltration
There is a great variety of channels and waterways, and
costs and capacities will vary from one to another
2.8.1 Data Collection Requirements
Data required for selection and design of a method of
surface water diversion are included in Table 2-10, along
with the reasons that the data are required.
2.8.2 Evaluation
Surface water controls will be required under certain cir-
cumstances. Cases when surface water controls might
be necessary would include:
Where an impermeable cap is used. Surface waters
should be directed away from such a cap so that
water does not pond directly above the contaminants.
If the contaminants are on the surface or contami-
nated soil is on or near the surface. Surface waters
flowing over the site could become tainted with the
chemicals present.
If infiltration through or to contaminated soils or
ground water is a significant problem.
2.8.3 Engineering Considerations for Implementation
There are several other issues to consider when deter-
mining whether or not to use surface water controls and
what type of surface water controls to use.
Grading is usually performed in conjunction with revege-
tation. The vegetation may further decrease infiltration
'28
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Table 2-10. Data Requirement for Runoff/ Run-On Control
Data Description
Purpose(s)
Source(s)/Method(s)
Topography
Local soil conditions
Availability of fill
Climate
To determine quantity of
runoff, necessity of
surface water controls
Determine usability as
fill; infiltration rates
To determine costs of
alternatives
To determine amount of
rainfall; amount of surface
water flow
Site survey
Topographic maps/
percolation testing
Site survey
National Weather
Service
and erosion, but without revegetation grading may in-
crease erosion. Periodic regrading may be necessary if
subsidence occurs.
Dikes and berms do not usually require a detailed design
before construction begins. It is important to remember,
however, that they are not usually used to control drain-
age from an area greater than 5 acres. In addition, they
must be designed so that there is sufficient grading for
runoff along the length of the dike or berm and the outlet
should be directly onto a stabilized area or drainage
structure.
Channels and waterways usually require a slightly
greater design effort than berms and dikes. They are
often designed according to the Manning formula which
can calculate the flow capacity of a given channel. Wa-
terways can be either permanent or temporary depend-
ing upon the design, and costs can vary significantly.
2.9 GAS VENTING
Gas venting, which may be active or passive, is applica-
ble to the containment (control of migration) of VOCs in
soil and, to a lesser extent, bedrock. The effectiveness of
a gas venting system is contingent upon soil type, soil
density, depth of ground water (saturated zone), and
specific gravity of the contaminant. Past applications
have primarily been in relation to control of methane gas
from landfills and interception of vapor plumes for gaso-
line or similar releases.
As a containment measure, the goal of gas venting is to
create a subsurface air flow pattern that confines hazard-
ous vapors to an area in which potential impacts are
minimal, or intercepts vapor flow before it reaches an
area where impacts are likely to be significant. The use
of vapor extraction systems is best suited for permeable,
unsaturated soils such as sand and gravel. Clayey soils
usually lack the conductivity necessary for effective va-
por extraction (U.S. EPA, 1990b).
The essential component of a gas venting system is a
gravel-packed vented trench or a series of vent wells.
The vent may be pumped (active) or may be simply open
to the atmosphere (passive). A passive vent system is
used, where permitted by state regulations, for situations
in which gas generated at a source area is lighter than
air (i.e., methane), which creates a driving force from the
source to-the atmosphere. Active vent systems are also
used for methane recovery from landfills and for the in-
terception of volatile contaminants migrating from a
source in the unsaturated zone of the soil. Typical
ranges of gas flow rates expected in natural in situ soils
can vary from 0.5 cubic feet per minute (cfm) to 2 cfm
per linear foot of well screen.
Additional vent system components include a vacuum
blower (for active systems), a manifold (to connect multi-
ple wells), off-gas treatment such as an enclosed flare or
carbon adsorption (to meet air quality criteria), a conden-
sate holding tank, and monitoring and control equipment.
Some of the benefits of gas venting are:
Rapid response time can be achieved
System is installed in situ and can be operated with
relatively little disturbance to other operations and
structures
The required equipment and services are readily avail-
able
Venting has relatively few secondary impacts, as no
chemicals or reagents are introduced into the soils
Some of the limitations to gas venting are:
29
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Venting can cause flow of unrelated vapor contamina-
tion, if present. :
Off-gas may require treatment; treatment residuals
may be RCRA-regulated waste. ;
The technique is only applicable to volatile contaminants
migrating in the vapor phase, and will not intercept free-
phase liquid contamination. However, continued venting
wll induce volatilization of the free phase. ]
2.9.1 Data Collection Requirements :
The data required for assessment and design of waste con-
tainment by gas venting are primarily related to the distribur
tion of the contaminant of concern and the ability of the
unsaturated zone to transmit vapor flow. In addition, some
environmental factors are also important in determining
whether gas venting should be pursued. Table 2-11 sumr
marizes the data requirements for implementation of this al-
ternative and relevant data collection methods. i
2.9.2 Evaluation
In most situations where vapors migrating through the
unsaturated zone have the potential'to adversely affect
human health or the environment, gas venting has the
potential to reduce risk. The following discussion as;-
sumes that initial evaluation of the site has indicated that
significant vapor concentrations are present, which have
potential to migrate in the vapor phase to a point of im-
pact (structures may be damaged or injury caused in the
case of explosive gases, or employees or local residents
may be harmed in the case of toxic gases). Under these
conditions, the relative economic feasibility of the tech-
nique depends on its projected cost as compared to
other stabilization measures. The cost of a gas venting
system depends on:
The number and depth of wells to be drilled and the
amount of associated piping ;
The flow rate and pressure at which active wells will
be pumped \
m The type and extent of off-gas treatment required :
The arrangement of wells and projected pumping rates
can, for the screening process, be estimated from per-
meabilities derived from soil descriptions and laboratory
testing. Rgure 2-7 shows the predicted extraction rates
for a range of soil permeabilities and applied vacuums,
assuming a well radius of 2 inches and a 40-foot radius
of influence during pumping. Similar results for other pa-
rameter values can be obtained from the formulae in
Johnson et al. (1990).
Pumping rates and expected discharge concentrations are
compared to applicable air discharge criteria to determine
what off-gas treatment is likely to be required. These data
are then used to develop order-of-magnrtude costs for kv
staUation and operation of the system. These costs can
then be compared to any other potentially applicable reme-
100-
10-
1 -
"E
.01
.001 -
.0001
Clayey Fine Medium Coarse
Sands Sands Sands Sands
1100
- 110
- 11
- 1.1
- 0.11
- 0.011
. 0.0011
.01 .1 1 10 100 1000
Soil Permeability (darcy)
Notes:
Well Radius = 2 inches
Radius of Influence = 40 ft
Pw = Pressure in Extraction Well
Figure 2-7. Soil Permeability versus Vapor Flowrate for
Several Values of Applied Vacuum (Adapted from John-
son et al., 1990).
dial costs to determine which technique is most feasible.
The data required for screening will typically be available
shortly after the initiation of RFA activities.
In order to develop the final design of a gas venting sys-
tem, accurate data regarding soil permeability and the
ability of air to move through the soil are required. One
method to determine air permeability is the air pumping
test. Air pumping tests are similar to ground-water pump-
ing tests used to determine aquifer yield, except that the
wells are screened above the ground-water table. A sin-
gle well is pumped with a vacuum blower at a known,
constant flow rate, and the resulting changes in air pres-
sure are measured in surrounding well(s). This informa-
tion is used to determine the range over which a well will
intercept vapor flow, as a function of pumping rate, and
is used to select a suitable vent (extraction well) system
geometry for containment and sizing of equipment.
The geometry and method to be used at a site are deter-
mined primarily by the thickness of the permeable zone
and the soil air permeability. Excavation conditions that
are not difficult, a shallow permeable zone, and high in
situ soil permeability all favor the use of trenching meth-
ods. A deep, highly permeable unsaturated zone and dif-
ficult excavation conditions favor the use of wells.
Pressurized systems (air injection) must be used with
caution, because contaminants are forced away from the
point of containment (injection well or trench) and can
then migrate toward other potential receptors.
30
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Table 2-11. Data Requirements for Gas Venting
Data Description
Purpose(s)
Source(s)/Method(s)
Physical Characteristics
Permeability
Depth of ail-
permeable zone
Moisture content
Determine radius of system
influence; estimate pumping
rate for active venting systems
Determine depth of system
necessary to contain or
intercept flow
Estimate effects of expected
moisture variations on air
permeability
Estimate from soil descriptions
Laboratory: Evans, 1965.
Field (pumping test):
Johnson et. al., 1990.
Soil boring program
ASTM D2216
Soil temperature
Estimate effect of temperature
changes on air viscosity and
resulting flow velocity
Appendix V of
U.S. Army (1972)
Soil type/gradation
Chemical Characteristics
Contaminated area
Phase distribution
of contaminant
Environmental Characteristics
Proximity of other
contaminant sources
Estimate flowrates in soil
Determine size and location
of system
Non-gaseous phases will not
be intercepted, but may be
affected
Determine whether vent system can
induce migration of additional
contamination on site, or .
controlled contamination off site
ASTM D422
Soil gas sampling
Soil borings/
monitoring wells;
literature study
Site survey and
historical records;
Soil sampling program
2.9.3 Engineering Considerations for
Implementation
The implementation of a soil-venting system includes
general site preparation, installation and operation, and
maintenance.
General preparations include making the site accessible
to equipment and materials and dust and volatile emis-
sions controls, if necessary.
During installation, waste removal and disposal in ac-
cordance with RCRA regulations will be required for drill-
ing and trenching operations in contaminated areas.
Waste may also be generated by off-gas treatment, i.e.,
condensate.
There are several issues concerning implementation,
which are as follows:
B Potentially hazardous vapor may not be contained, or
may be driven to unanticipated receptors, when using
pressurized wells or trenches
Numerical modeling has shown that in many typical
landfill situations, venting may not be effective if the
vents do not penetrate the full thickness of the unsatu-
rated permeable zone
31
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Extraction well systems should be installed such that
the influence of each well overlap; numerical modeling
has shown that inadequately spaced wells can aggra-
vate vapor migration (Moore et al., 1982) ;
Active systems using a collection manifold should be
designed to drain condensate to a central collection
point, and to prevent freezing in colder climates :
Passive vents are not generally effective except along
the perimeter of landfills where methane is generated
Gas treatment and condensate collection produce
waste that may require special handling and must
comply with RCRA disposal regulations
Gas can be collected for use after extraction, i.e.,
methane collected from a landfill can be used to gener-
ate electricity or steam if at adequate concentrations
(i.e., 35 to 55 percent)
2.10 HYDRAULIC FRACTURING
Hydraulic fracturing refers to the injection of fluid into soil
or rock under pressure to create fractures, and thereby
increasing the hydraulic conductivity. A proppant, which is
generally a permeable material such as sand, is injected
with the fluid to keep the fractures open after the pressure
is released. This technology is not a containment or treat-
ment technology in itself; however, it may be used to
modify a formation of low hydraulic conductivity in ordef
to permit or enhance other technologies requiring a per-
meable medium such as pump-and-treat, vacuum extrac-
tion, soil flushing, or bloremediation.
In the case of hydraulic fracturing of bedrock, much ex-
perience has been gained from the oil industry, where
this technology was initially developed. The hydraulic
fracturing of soils is a less-developed technology. The
Center Hill Research Facility at the University of Cincin-
nati has an ongoing project to test the feasibility of hyl-
draulically fracturing soils and using the newly formed
fractures to enhance other remediation technologies,
particularly bioremediation. To date, feasibility of hydrau;-
lic fracturing has been demonstrated only in an unsatuL
rated glacial till soil in Ohio.
A potential benefit of hydraulic fracturing is that it can be
used to develop a larger framework of interconnected
pore space, and consequently enhance the zone of influ-
ence of a particular remediation technology. This technolr
ogy, when further developed and proven, may be useful
for remediating soils in cases where contaminants have
migrated by slow advection or diffusion into materials with
very low hydraulic conductivities. Its application in cases
of contamination of bedrock aquifers, in which it is used
to increase the yields of extraction wells, has been well
demonstrated by similar applications in potable water well
construction.
The principal drawback of using this technology in soils is
that it has not yet been demonstrated in a wide range of
soil types or at any waste sites.
2.10.1 Data Collection Requirements
Since this technology is still in the developmental stages for
applications in soils, the data collection requirements are not
well documented. Many of the requirements in Table 2-12
are common to containment or pump-and-treat technologies
that would be enhanced by hydraulic fracturing.
2.10.2 Evaluation
The first steps in the evaluation of the applicability of this
technology are (1) determination of the need to increase
hydraulic conductivity and (2) determination of the con-
tainment or treatment technology that is going to be used
after enhancement by hydraulic fracturing. The useful-
ness of hydraulic fracturing has been well documented in
increasing well yields in rock. Thus, its usefulness in en-
hancing pump-and-treat systems in low-yielding rock for-
mations is that larger yields may result and a more
widespread area may be impacted by a single well. This
will result in fewer overall wells and shorter cleanup times.
Its usefulness in enhancing bioremediation of saturated
and unsaturated soils and ground water, soil flushing, and
vacuum extraction is potentially the same but has yet to
be demonstrated.
The orientation of the fractures created by hydraulic frac-
turing depends largely on the in situ state of stress in the
material to be fractured. The plane of fracture will be per-
pendicular to the direction of least principal compression
(U.S. EPA, 1991b). The reference cited contains a de-
tailed explanation of the theoretical analysis of fracture
propagation.
The evaluation that has been performed at experimental
fracturing sites in soil has consisted of excavating test pits
or trenches around the hole from which the fractures were
propagated to determine their configuration and width. It
is conceivable that the evaluation of this process at any
given site would include a pilot study to determine if the
actual fracture pattern matches the predicted one.
2.10.3 Engineering Considerations for
Implementation
The applicability of hydraulic fracturing to remediation of
waste sites through the enhancement of containment or
treatment technologies such as bioremediation, pump-
and-treat, vacuum extraction, or soil flushing is dependent
on site conditions. Although the method has been widely
demonstrated to increase well yields in bedrock and
therefore to be applicable to low-yielding pump-and-treat
enhancement, it has not yet been widely tested in soils
alone or with the other technologies. A potential drawback
to this technology in soils and bedrock includes creating
undesirable pathways of contaminant migration into pre-
viously unimpacted areas. Also, it does not result in a
32
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Table 2-12. Data Requirements for Hydraulic Fracturing
Data Description
Purpose(s)
Source(s)/Method(s)
Bedrock type, degree
of fracturing and
weathering
Determine applicability
of method to bedrock
conditions
Geologic maps, bedrock out-
crop mapping, rock coring
of bedrock in borings,
bedrock trench mapping
Depth to aquifer/
water table
Select appropriate
depths for fractures
Hydrogeologic maps, observa-
tion wells, boring logs,
piezometers, geophysical surveys
Types, thicknesses,
and extents of
saturated and
unsaturated sub-
surface materials
Design fracturing system
Hydrogeologic maps, surfi-
cial geology maps/reports,
boring logs, geophysics
Hydraulic conduc-
tivities and stora-
tivities of subsurface
materials
Determine need for
hydraulic fracturing
Pumping tests, slug tests,
laboratory permeability
tests
Contaminant
concentrations and
areal extent
Locate and design
fracturing system
Water quality data
NAPL density/
viscosity/solubility
Predict vertical dis-
tribution of contamina-
tion, design fracturing
system
Literature
completely permeable medium but instead creates path-
ways by which otherwise inaccessible fractures or soil
masses may be reached by remedial technologies.
2.11 REFERENCES
Bicknell, B.R. 1984. Modeling Chemical Emissions from
Lagoons and Landfills, Final Report. U.S. EPA Environ-
mental Research Laboratory, Athens, CA.
Cole, C.R. 1982. Evaluation of Landfill Remedial Action
Alternatives through Groundwater Modeling. Proc. of 8th
Annual Research Symposium on Land Disposal of Haz-
ardous Waste. EPA-600/9-82-002.
Colorado State University. 1988. Digital Operational Man-
agement Model of the North Boundary System at the
Rocky Mountain Arsenal Near Denver Colorado. Techni-
cal Report No. 16, Dept. of Civil Engineering.
Dass P. et al. 1977. Leachate Production at Sanitary Land-
fills. Proceedings of the American Society of Civil Engineers.
Journal of the Environmental Engineering Division.
Eastman Christensen Company. 1991. Horizontal Well-
bore System for Ground-Water Remediation. Informa-
tional Circular.
Fenn, D.G. et al. 1975. Use of the Water Balance Method
for Predicting Leachate Generation from Solid Waste Dis-
posal Sites. Report SW-168. U.S. EPA.
Hantush, M.S. 1964. Hydraulics of Wells. In: Advances in Hy-
droscience (V.T. Chow, ed.). Academic Press, New York, NY.
33
-------
Hanlush, M.S. and I.S. Papadouplos. 1962. Flow of
Groundwater to Collection Wells. Proc. Am. Soc. Div. En-
grs., pp. 221-244.
Johnson, P.O., C.C. Stanley, M.W. Kemblowski, D.L. By-
ers, and J.D. Colhart. 1990. A Practical Approach to the
Design, Operation, and Monitoring of In Situ Soil Venting
Systems. Groundwater Monitoring Review, Vol. 10, No. 2.!
Kapack D.S., B.B. Looney, C.A. Eddy, and T.C. Hazen.
1991. Innovative Groundwater and Soil Remediation. In
Situ Air Stripping Using Horizontal Wells. NWWA Third
Outdoor Action Conference Proceedings, pp. 47-58. :
Karlsson, H. and R. Brtto. 1990. New Horizontal Wellbore
System for Monitor and Remedial Wells. HMCR111th Na-
tional Superfund Conference Proceedings, pp. 357-362.
Langseth, D.E. 1990. Hydraulic Performance of Horizon-
tal Wells. HMCRI 11th National Superfund Conference
Proceedings, pp. 398-408.
Lohman, S.W. 1972. Ground Water Hydraulics. U.S. Geo-
logical Survey Professional Paper 708. U.S. Government
Printing Office.
Moore, Charles A., Iqbal S. Rai and John Lynch. 1982.
Computer Design of Landfill Methane Migration Contra).
Journal of the Environmental Engineering Division,
ASCE.Vol.108.No. EE1.
Moore, Charles A., Iqbal S. Rai, and Ayad A. Alzaydj.
1979. Methane Migration Around Sanitary Landfills. Jour-
nal of the Geotechnical Engineering Division, ASCE. Vol.
105, No. GT2.
Osiensky, J.L. 1983. Groundwater Withdrawal Schemejs
for Uranium Mill Waste Disposal Sites. Groundwater
Monitoring Review.
Quince, J.R. and G.L Gardner 1982. Recovery and
Treatment of Contaminated Groundwater. Groundwater
Monitoring Review.
Spooner, P.A...R.S. Wetwell, and W.E. Grube, Jr. 1982.
Pollution Migration Cut-Off Using Slurry Trench Construc-
tion. Conference Proceedings, Management of Uncon-
trolled Hazardous Waste Sites.
U.S. Air Force. 1971. Dewatering and Groundwater Con-
trol for Deep Excavations. TM-5-818-5 NAV FAC P-418,
AFM 88-5 Chap. 6.
U.S. EPA. 1991 a. Conducting Remedial Investiga-
tion/Feasibility Studies for CERCLA Municipal Landfill
Sites. EPA/540/P-91/001.
U.S. EPA. 1991 b. Feasibility of Hydraulic Fracturing of
Soil to Improve Remedial Actions. Risk Reduction Labo-
ratory. Cincinnati, OH.
U.S. EPA. 1990a. Ground-Water Handbook. Volume I:
Ground Water and Contamination. EPA/625/6-90/016a.
U.S. EPA. 1990b. Handbook on In Situ Treatment of Haz-
ardous Waste-Contaminated Soils. EPA/540/2-90/002.
U.S. EPA. 1989. Technical Guidance Document: Final
Covers on Hazardous Waste Landfills and Surface Im-
poundments. EPA/530-SW-89-047.
U.S. EPA. 1987. Technology Briefs: Data Requirements for
Selecting Remedial Action Technology. EPA/600/2-87-001.
U.S. EPA. 1985a. Modeling Remedial Actions at Uncon-
trolled Hazardous Waste Sites. EPA/540-2-85/001.
U.S. EPA. 1985b. Handbook for Remedial Action at
Waste Disposal Sites. EPA/625/6-85/006.
U.S. EPA. 1984a. Slurry Trench Construction for Pollution
Migration Control. EPA 540/2-84-001.
U.S. EPA. 1984b. Compatibility of Grouts with Hazardous
Wastes. EPA 600/2-84-015.
U.S. EPA. 1984c. The Hydrologic Evaluation of Landfill Per-
formance (HELP) Model, Volume 1. EPA/530-SW-84-009.
U.S. EPA. 1983. Methods for Chemical Analysis of Water
and Wastes. EPA 600/4-79-020.
Weston, Inc. 1989. Remedial Program Evaluation, Gilson
Road Site. Nashua, NH. Prepared for NHDES, February.
34
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CHAPTER THREE
Soils Treatment Technologies
Treatment of contaminated soils is conducted as a
means of source control to reduce volume, toxicity, or
mobility of contaminants. A specific goal of treating con-
taminated soils is to prevent contaminants from leaching
into ground water, migrating off site, and threatening
drinking water supplies or other significant resources.
Soils may be treated in situ (i.e., in place) or by excavat-
ing and then treating. The decision to treat in situ or to ex-
cavate depends on the areal extent, depth and volume of
contamination, type and concentration of contaminants
present, soil characteristics, site hydrogeology and other
site characteristics.
The selection of the appropriate treatment technology is
dependent upon the properties of the contaminant and
the physical and chemical properties of the soil matrix. In
situ treatment may be physical, chemical, thermal, bio-
logical, or a combination thereof. A thorough knowledge
of the site characteristics is essential in determining
whether in situ treatment can be utilized and estimating
the success of the selected treatment technology. An ad-
vantage to in situ treatment is that the RCRA land dis-
posal restrictions (LDRs) are not applicable.
In situ treatment is preferred over soil excavation at sites
where contamination is widespread, in both area and
depth, or where the contaminated material is not easily
excavated. In order for in situ treatment to be effective,
however, the site characteristics and soil properties must
allow for the injection of air, steam, water, or treatment
chemicals, as necessary, to implement the technology
under consideration. In many cases, it is desirable to ex-
cavate contaminated soil because it is an active source
for ground-water contamination. Excavation should be
considered under the following conditions:
The need to remove the contaminant source is imme-
diate
The site conditions prohibit or limit the effectiveness of
in situ treatment
The treatment technology under consideration requires
or would be enhanced by excavation (i.e., solidifica-
tion/stabilization)
Figure 3-1 presents the technologies applicable for cer-
tain site conditions and contaminants. This figure is pro-
vided as a preliminary screening tool, to direct the reader
to the technologies that may be considered for a specific
set of conditions.
The treatment technologies described in the following
subsections may be used independently, in combination
with each other, or in series with other technologies. The
technologies presented herein have been included since
they are applicable to a number of potential site condi-
tions that could require a stabilization action. This does
not imply, however, that other technologies could not be
considered for soils treatment. All of the stabilization
technologies presented herein are for in situ treatment,
with the exception of the solidification/stabilization proc-
ess, which may be applied to either excavated or in situ
soils.
3.1 SOLIDIFICATION/STABILIZATION
The solidification/stabilization (s/s) process, sometimes
referred to as immobilization, fixation, or encapsulation,
uses additives or processes to physically or chemically
immobilize the hazardous constituents of a contaminated
soil.
The basic s/s procedure involves three steps: (1) the mix-
ing of a reagent with the soil, (2) the curing of the mixed
product, and (3) the storage or landfilling of the treated
soil. Mixing of the soil and reagent can occur in situ either
by using a backhoe to apply and mix additives, or by us-
ing a more sophisticated injector device with augers to in^
ject the reagent into the soil and a paddle assembly to
mix the materials. Due to increasingly stringent volatile
and paniculate collection requirements, in-vessel mixing
is often preferred.
Vessel-type mixers include drums, pug mills, and ribbon
blenders. Both mobile and fixed installation systems with
emissions control devices are available for mixing and
curing of s/s wastes. Curing of the s/s waste may take
place in situ, in the mixing vessel, or in a separate stor-
age area/facility. The treated soil is subsequently land-
filled or backfilled on site.
Solidification/stabilization processes are applicable to
soils contaminated with metals and other inorganics, in
addition to nonvolatile and semivolatile organic com-
pounds. The processes are not currently viewed by EPA
as applicable for remediation of soils contaminated solely
with volatile organic compounds, because these com-
35
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Site Conditions
Soil Permeability High
Low
Depth to Contamination <1 5'
>15'
Highly Organic Soils
Hetergeneous Soils
Contaminants
Nonhalogenated Volatile Organics
Halogenated Volatile organics
Semivolatile Organics
PCBs/Pesticides/Dioxins
Metals
Other Inorganics
O
O
X
O
O
X
X
O
O
X
0
X
x
O
O
0
X
O
O
X
X
X
O
X
O
0
X
O
O
O
X
0
O
X
X
O
X
X
X
X
O
O
X
X
O
X
X
X
9
ซ
9
X
O
ซ
O
O
X
X
X
e
X
O
O
O
e
X
X
X
X
References: See References for Individual Technologies Applicable
O Potentially Applicable
1 x Not Applicable
Figure 3-1. Applicability of Soils Treatment Technologies.
pounds will be released during the mixing and curing
process (U.S. EPA, 1991).
The benefits of using this technology include: \
Additives and reagents are widely available and rela-
tively inexpensive I
The resulting solidified/stabilized material may require
little or no further treatment if proper conditions are
maintained |
Leaching or mobility of contaminants is greatly
reduced I
Soils contaminated with both metals and semivolatile
or nonvolatile organic compounds may be treated in
one step
The limitations of s/s technologies include:
S/s technologies do not destroy contaminants
Volume of treated material may increase significantly
with the addition of reagents
Delivery of reagent to the subsurface and achieving
uniform mixing for in situ treatment may be difficult
36
-------
Emission of volatile organic compounds and particu-
lates may occur during mixing procedures, requiring
extensive emission controls
In situ solidification of sensitive areas, e.g., wetlands,
may inhibit their restoration for future use
Compliance with RCRA LDRs must be demonstrated if
s/s materials are to be landfilled
3.1.1 Data Collection Requirements
Data regarding both physical and chemical charac-
teristics of the contaminated soil should be obtained to
assess the feasibility and design of the s/s process. The
physical characteristics of the soil will determine handling
requirements; ;the chemical characteristics will determine
the degree of hazard in handling or treatment proce-
dures, identify s/s process inhibitors so that pretreatment
alternatives can be developed, and assess the compati-
bility of the soil and s/s processes. Information regarding
the environmental setting of the site to be treated and
planned future land use should also be collected. Ele-
ments of the environmental setting of a contaminated site
affect the design of a s/s treatment scheme, particularly if
the waste is treated in situ or if the treated waste is back-
filled on site. Table 3-1 summarizes the data require-
ments for the s/s technology.
3.1.2 Evaluation
The effectiveness of proposed s/s processes is waste and
site specific and cannot be predicted without treatability
testing. At a minimum, the selected s/s treatment process
should be evaluated on a bench-scale study to determine:
Compatability of the soil to be treated with s/s processes
Safety problems in handling the contaminated soil
Soil uniformity and mixing and pumping properties
Volume increases associated with processing
Development of processing parameters and the level
of processing control
Although bench-scale studies will yield such information,
a pilot-scale study of the process will provide more accu-
rate, realistic testing and information to predict the feasi-
bility of the proposed s/s treatment process.
Chemical analyses of the soil are needed to determine if
the contaminants are amenable to treatment by s/s proc-
esses. Most inorganics and semivolatile organic com-
pounds can be effectively immobilized, but s/s
technologies are not applicable to soils contaminated
solely with volatile organic compounds.
Chemical analyses are also important to determine
whether the soil contains constituents that could inhibit
the effectiveness of s/s processes. Potentially interfering
constituents are specific to the particular s/s process under
evaluation, and .include oil and grease, halides, and sulfates.
The use of s/s technologies for soil remediation is most
appropriate for soils contaminated with only inorganics,
or with inorganics and low levels of semivolatile or non-
volatile organic compounds. The use of these technolo-
gies for treatment of soils containing high levels of
organics is not as well developed.
Safety problems associated with handling the soil during
the s/s process include fuming, heat development, vola-
tilization of organic contaminants, and dust evolution. The
generation of gases or high heats of reaction or hydration
will be identified in bench and pilot studies.
Variations in the soil sample can alter mix viscosity and
cause mixing or pumping problems. Pilot studies should
be used to evaluate the effectiveness of various types of
mixers and pumps.
An increase in volume of a treated soil over the untreated
soil is normally associated with the s/s procedure. The
degree of volume increase will be best predicted by the
results of pilot testing. Processing parameters such as
mix ratios, mix times, setting times, and curing conditions
can be optimized during bench and pilot testing.
In addition to a process evaluation, the stabilized/solidi-
fied product must also be evaluated to determine the ef-
fectiveness of the s/s process. The following criteria
should be included in the evaluation:
teachability of hazardous constituents
Hydraulic conductivity
Compressive strength
Freeze/thaw and wet/dry properties
The teachability of contaminants from the treated soil
should be reduced due to chemical or physical binding
with the additive.
The hydraulic conductivity of the s/s product is a measure
of the rate of movement of water that can pass through
the waste. Thus, hydraulic conductivity'should be low to
minimize the quantity of water capable of contacting the
treated soil.
The compressive strength of the treated soil should be
sufficient to support structures planned for future uses of
the backfilled site. The required compressive strength of
the treated soil should be at least 50 psi (U.S. EPA,
1986a).
The treated soil should retain its integrity when subjected
to expected freeze-thaw and wet/dry cycles. Fracturing of
the treated soil exposed to these cycles increases the
surface area over which water contact can be made, thus
increasing the potential for contaminant leaching.
Examination of the treated soil on a microscopic scale by
methods such as scanning electron microscopy or x-ray dif-
fraction can characterize crystallization contacts, porosity,
uniformity, and degree of hydration of the s/s product.
37
-------
Table 3-1. Data Requirements for Solidification/Stabilization Technology
Data Description
Purpose(s)
Source(s)/Method(s)
Physical Characteristics of Waste
Percent moisture
or water content
Bulk density
Grain size
distribution
To identify pre|treatment
requirements (i.e., settling
or drying) or to design
solidification procedures
j
To convert weight to volume in
materials handling calculations
i
I
To design remedial actions
and identify pretreatment
requirements;! very fine-grain
wastes create! dust problems
and lower the ultimate
strength of stabilized cement
composites; bpulders or debris
must be crushed or removed.
ASTM D2216-80
ASTM D2937-83, D1556-82
D2922-81,03402.025
Appendix V of
ASTM D422-63
Cone Index
To determine the ability of in-
place wastes to support a load
Sowers and Sowers
(1970)
ASTMD3441-79
Unconflned com-
presslve strength
Chemical Characteristics of Waste
teachability of
hazardous
constituents
Bulk chemical
analyses Including
organic solvents and
oils, solid organics,
acid wastes, oxtdizers,
sulfates, halldes,
heavy metals, and
radioactive materials
To determine the load-bearing
capacity of the waste
To determine the hazard
due to toxicity of a waste
To identify s/s inhibitors
and to assess the compati-
bility of wastes and s/s
processes
ASTM D2166-85
Several leaching and
extraction tests are
available (U.S. EPA, 1989a)
U.S. EPASW-846(1986c)
U.S. EPA (1979)
38
-------
Table 3-1 (Continued)
Data Description
Purpose(s)
Source(s)/Method(s)
Environmental Setting
Water table
elevation
To determine whether
dewatering of waste is
necessary in in situ
applications
Use existing local
wells or install
observation wells
and/or piezometers to
determine average and
maximum water table
elevations at the site
Climate
Frost line depth
Floodplain
locations
To determine if tempera-
ture is suitable for
proper curing
To assess whether back-
filled treated waste will
undergo freeze-thaw cycles
To avoid backfilling
treated waste within a
floodplain to minimize
contact with water
Contact National
Weather Service or
obtain values of average
temperature and temperature
range for the site
Consult local Soil
Conservation Service
Refer to Flood
Insurance Rate Map for
site area; consult town
planning bureau or state
environmental agency
These microstructural elements affect the durability and
teachability of the s/s product.
3.1.3 Engineering Considerations for
Implementation
The implementation of a selected solidification/stabiliza-
tion process typically involves the following steps:
Pretreatment of contaminated soils
Removal of contaminated soils
Storage of materials
Mixing and metering of materials
Landfilling of the s/s product
Pretreatment of a soil is often required to facilitate the s/s
process or to increase its effectiveness and/or safety.
Pretreatment may include the removal of boulders, dewa-
tering of the waste source area, neutralization or homog-
enization of the soil, or removal of volatile organic
compounds through processes such as soil venting.
Soil removal is an implementation step required at sites
where in situ mixing is not conducted. Typical earth-mov-
ing equipment (backhoes, drag lines, bulldozers, and
front-end loaders) are usually utilized, although specific
equipment depends on the physical nature of the soil.
Removed soils must be transported to the treatment facil-
ity which may be located on or off site. Typical transport
systems include pumps/hoses for liquid wastes, conveyor
belts for soils treated on site, screw augers, and dump
trucks. Vehicles transporting soils off site must be lined to
prevent the leakage of wastes.
Storage of materials is required for both in situ and exca-
vation type s/s treatment processes. Storage areas must
be allotted for soils awaiting treatment, chemical re-
agents, and treated soils. Sufficient space is needed so
that the treatment operation is not delayed due to the lack
of materials. The storage space must also be dry and
protected from wind tp avoid dispersion of materials.
Chemical reagents may require tanks, bins, or hoppers;
waste materials may require lined pits and water collec-
tion systems.
39
-------
Mixing of the soil and reagent may occur in situ or in-ves-
sel and is probably the most important step in implement-'
ing an s/s process. In situ mixing is commonly
accomplished using a backhoe or an injector-type mixeri
The volume or weight of a chemical reagent must be
measured before mixing is begun. Vessel-type mixers
such as pug mills or ribbon blenders may have a special!
ized metering system to add reagents during mixing*
Drums may be used as mixing vessels in small-scale
treatment systems.
The final implementation process is the on or off site land
disposal of the stabilized/solidified soils. Compliance wit^i
RCRA LDRs must be demonstrated for soils contamil-
nated with a restricted RCRA hazardous waste. The
treated soil may be transferred to the final disposal area
prior to or following the curing period. If the treated soil is
fluid-like, it may be pumped to an excavated area on site
or to dump trucks for transportation to an off site landfill.
Otherwise, it may be backfilled using traditional earth-
moving equipment.
3.2 SOIL FLUSHING
Soil flushing is used for removal of a number of organip
and inorganic materials from vadose zone soils. A vari-
ation of soil flushing, referred to as chemical extraction,
may be used to remove non-water soluble organics from
the saturated zone. '
Soil flushing involves the addition of a solvent or surfac-
tant to contaminated soil to enhance contaminant mobil-
ity. The contaminants are then recovered in the ground
water by strategically placed extraction wells and pumped
to the surface for treatment. Soil flushing is most applica-
ble when soils must be remediated but other technologies
such as vacuum extraction, bioremediation, or physical
removal (i.e., excavation) are not feasible. High water ta-
bles, deep contamination, and high-permeability soils that
require dewatering are conditions in which these tech-
nologies would be less feasible. Soil flushing is not feasi-
ble for soils with low-hydraulic conductivities (e.g., less
than 1 ft/day) and for strongly adsorbed contaminants
(e.g., RGBs, dioxin). The addition of chemicals to the
flushing solution that will increase contaminant mobility
are necessary, if strongly adsorbed, hydrophobic con-
taminants are present in the soil. The extraction of
strongly adsorbed contaminants may not be desirable for
a stabilized action, however, unless there is an imminent
threat to human health and the environment. The mor(e
permeable the soil, the more water that can be flushecl
through the soil and the more practicable this technology.
Soil flushing strategies can be incorporated into pump-
and-treat or containment systems to accelerate the con-
taminant removal processes. Soil flushing can be
accomplished using sprinkling systems or, more aggres-
sively, by flooding the contaminated area. Chemical ek-
traction involves extracting ground water, amending j it
with solvents and/or other chemicals, and reinjecting it at
strategic locations into the aquifer (U.S. EPA, 1990b).
With any soil flushing system, proper controls must be in-
corporated to prevent migration of extractant-contaminant
mixtures.
The flushing solution to be used at a site depends on the
type of contamination present. Flushing solutions may in-
clude water, acidic aqueous solutions (sulfuric, hydro-
chloric, nitric, phosphoric, and carbonic acids), basic
solutions (e.g., sodium hydroxide), surfactants (e.g., al-
kylbenzene sulfonate), chelating agents, oxidizing
agents, or reducing agents. Water can be used to extract
water-soluble or water-mobile constituents. Acidic solu-
tions are used for metals and certain organic constituents
(including amines, ethers, and anilines) that are soluble
in an acidic environment (U.S. EPA, 1990b). Surfactants
can be used for hydrophobic organics, such as oils and
petroleums (U.S. EPA, 1990b and c).
The level of treatment that can be achieved will vary de-
pending on the contact of the flushing solution with waste
constituents, the appropriateness of the solutions for the
wastes, the soil adsorption coefficients of the waste parti-
tioning coefficients, and the hydraulic conductivity of the
soil. This technology should produce the best treatment
results in highly permeable soils with low organic content.
Despite the varying level of treatment accomplished by
soil flushing, however, once the waste components have
been removed from the soil, results are not reversible.
Soil flushing may be used as a pretreatment for, or in
combination with, bioremediation. As a pretreatment
step, soil flushing may be used to remove inhibitory com-
pounds or reduce contaminant levels, making the soil
media more amenable to biological activity. In combina-
tion with bioremediation, the flushing solution can be
amended with nutrients to enhance biological activity.
The benefits of using soil flushing include:
Removal of contaminants is permanent
Removal of soils is not necessary
The technology is easily applied to permeable soils
The limitations of soil flushing technologies include:
The technology introduces potential toxins (the flush-
ing solution) into the soil system. Therefore, contain-
ment may be needed.
Physical/chemical properties of the soil system may be
altered because of the introduction of the flushing
solution.
A potential exists for solvents to transport contami-
nants away from the site into uncontaminated areas.
Therefore, containment may be needed.
A potential exists for incomplete removal of contami-
nants due to heterogeneity of soil permeability.
Contaminants are not destroyed. Onsite treatment is
required to remove contaminants from extracted sol-
vents.
40
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Flushing agents usually cannot be recycled.
It may take a long period of time for remediation below
cleanup standards to be achieved.
3.2.1 Data Collection Requirements
Both physical and chemical characteristics of the waste
and site soils must be known to assess the feasibility of
soil flushing. In addition, hydrogeological data are
needed for placement of extraction wells and, in the case
of chemical extraction, reinjection wells. Data collection
requirements are presented in Table 3-2.
3.2.2 Evaluation
Several factors to consider when evaluating a treatment
technology for stabilizing a site include (1) how well the
technology will work on the contaminated media; (2) how
long the stabilization process will take using this technol-
ogy; (3) the residuals produced by the process; and
(4) the cost of the technology as compared to a technol-
ogy capable of producing comparable results.
Once it has been established that site hydrogeology is
amenable to soil flushing, and the nature and solubility of
contaminants are known, the technology can be evalu-
ated in more depth. How well the technology will work
and the length of time it will take depends to a great de-
gree on the flushing solution selected. An additional fac-
tor in determining duration of stabilization is the number
of pore volumes, or flushes, required to remove contami-
nation. The residuals produced will depend on the type of
treatment required for recovered flushing solution. For in-
stance, an acidic flushing solution used to remove metals
from soil could be treated by pH adjustment, metals precipi-
tation and clarification, or reacidification and then reused. In
this case, a metal hydroxide or sulfide sludge may be pro-
duced. If the solution cannot be treated and reused, it will
require disposal. Cost of the process will depend greatly on
the flushing solution used and whether or not it can be re-
covered and recycled, the method of flushing (e.g., extrac-
tion and reinjection or flooding), number of pore volumes
necessary, and the controls required.
3.2.2.1 Treatability Testing. The optimum flushing solu-
tion(s) for a particular site should be determined experi-
mentally before proceeding on to full-scale remediation.
This can be done by conducting shake flask tests or col-
umn tests. The flask tests are conducted by filling 500-
ml_ Erlenmeyer flasks with contaminated site soil and
various flushing solutions or surfactants, selected based
on the nature of the contaminants, and mixing to allow
contact between the solution and the soil. Flask tests do
not simulate in situ treatment, but provide screening infor-
mation to select a potential flushing solution.
In situ conditions are best simulated at the bench-scale
by column tests rather than flask tests. A column test is
performed by packing contaminated soil into a column
and applying the flushing solution to the soil. Replicate
columns can be used to determine the number of pore
volumes required to remove contamination with a particu-
lar flushing solution. Replicate columns can be sacrificed
after different numbers of flushes, so that soil analyses
may be performed. The analyses provide data on con-
taminant removal as a function of the number of pore vol-
umes flushed.
3.2.3 Engineering Considerations for Implementation
Information on the design and performance of full-scale
soil flushing systems is limited. There are a number of
factors that must be considered for the design and imple-
mentation of a soil flushing system.
3.2.3.1 Introduction and Recovery of the Flushing
Solution. How the solvent or surfactant is to be applied
to the contaminated media is a critical part of system de-
sign. A sprinkling or, more frequently, a flooding system
must be designed to ensure saturation of the soil with the
flushing solution without spreading the contamination be-
yond the area of concern. An in situ chemical extraction
system must include strategically placed injection wells. Both
soil flushing and chemical extraction systems must include a
ground-water extraction system designed to hydraulically
capture the flushing solution and the contaminants. Vadose
zone and ground-water flow modeling may provide useful in-
formation for designing these systems.
3.2.3.2 Hydraulic Controls. For some systems, particu-
larly those that involve flooding a contaminated area, a
ground-water extraction system may not be sufficient to
prevent migration of contamination from the work area
during the flushing process. Actual hydraulic barriers,
such as slurry walls (see Section 2.2), may be necessary
to minimize or prevent migration.
3.2.3.3 Ground-Water/Flushing Solution Treatment.
Once the flushing solution has been recovered, treatment
for the removal of contaminants and, if necessary, sepa-
ration of the ground water and the solvent or surfactant
must be accomplished. The type of treatment will depend
not only on the nature of the contaminants but also on
the type of flushing solution. It is not possible to separate
most surfactants from water as easily as to separate
other flushing agents. Excess surfactant storage capacity
and the method and cost of disposal of spent surfactant
must then be considered during design. Disposal of treat-
ment system residuals must also be considered.
3.2.3.4 System Performance. A program should be de-
veloped to monitor system performance during operation.
The program should include sampling and analysis of
both the remediation area and the bordering areas. This
will provide data on whether contamination is migrating
as a result of the flushing process and alert operators
when system modifications may be required, as well as
provide data on changes in contaminant concentrations
in the remediation area. The monitoring program may in-
clude both sampling and analysis, and ongoing vadose
zone and ground-water flow modeling, using the sam-
pling results to continually update the models.
41
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Table 3-2. Data Requirements for Soil Flushing and Chemical Extraction Systems
Data Needed
Purpose
Collection Method
Contaminant
characterization
Necessary for selection of
flushing solutipn(s)
U.S. EPA
SW-846(1986d)
Equilibrium
partition coefficient
or octanol/water
coefficient, Kow,
or water solubility
Aqueous
solubility
Unfavorable separation coefficient
will require excessive volumes
of surfactant; also used to
select flushing solution
K < 10: amenable to natural flushing
ow !
K =1000: somewhat amenable
ow ' m
K > 1000: requires a surfactant
ow '
Used to determine whether
contaminants can be flushed
with water
Values available in
references such as
U.S. EPA, 1986d
Values available in
references such as
U.S. EPA, 1986d
Soil mapping
Used to determine variable soil
conditions that; could result in
inconsistent flushing
Soil sampling
Hydraulic
conductivity,
ground-water flow
pattern
Organic content
of soil
Ground water must permit recap-
ture of flushed| contaminants
,and flushing splutions
High organic content could
inhibit desorptlon of
contaminants
Slug tests, pumping
tests
U.S. EPA
SW-846(1986d)
(1) U.S.EPA,1990a.
3.3 BIOREMEDIATION
Bioremediation is the process of using bacteria to biode-
grade organic compounds in soils. Under favorable con-
ditions, microorganisms may be capable of completely
degrading many organic compounds into carbon dioxide
and water or organic acids and methane.
The applicability of bioremediation depends on the biode-
gradability of site contaminants. Petroleum compounds,
such as gasoline and diesel fuel, are known to be readily
biodegradable. Other biodegradable contaminants in-
clude alcohols, phenols, esters, and ketones. Chlorinatejd
compounds become more difficult to biodegrade as the
number of chlorine molecules increases. The biodegra-
dation rate, or half-life, of large, heavily chlorinated corp-
pounds such as RGBs is very slow. This makes
bioremediation an impractical stabilization technology for
soil contaminated with these compounds.
Bioremediation should not be considered as a stabiliza-
tion measure if the soil contains high levels of metals and
is not first pretreated, or if the soil is highly impermeable.
Bioremediation of contaminated soils is accomplished by
the degradation of specific organic constituents, or "par-
ent" compounds, to a number of intermediate com-
pounds. It is a stepwise process, which may involve
many enzymes, many species of organisms, and many
intermediate compounds before the parent compound is
mineralized.
Mineralization is the complete degradation of organic
compounds under aerobic conditions to CO2, hteO, inor-
ganic compounds, and cell proteins or, under anaerobic
42
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conditions, to organic acids, methane, and/or hydrogen
gas. Under normal degradation conditions, a constitu-
ent may not be completely mineralized but may be
transformed to intermediate products. These intermedi-
ate products may be just as hazardous as the parent
compound. The goal of controlled onsite bioremediation
is degradation of the parent compound to yield prod-
ucts which are not hazardous to human health or the
environment.
This handbook focuses on land-based bioremediation
methods, with soil itself used as the treatment medium, in
contrast to reactor-based systems. Particular emphasis is
on in situ treatment. Methods requiring excavation of the
contaminated soil are not addressed since the effort re-
quired for implementation may not be appropriate for an
interim measure. The bioremediation methods to be dis-
cussed, due to their applicability for stabilization in place,
include land farming and in situ treatment.
Landfarming involves the aeration of oil and other haz-
ardous materials in soil and sludges by tilling or other cul-
tivation methods, with the addition of nutrients. This
method has been used by the oil refining industry for the
disposal of oily sludges for many years. The methods can
be applied in situ, where soil contamination is relatively
shallow. Addition of microbial cultures can be used to
augment the indigenous microbial population and speed
up the rate of biodegradation.
In situ bioremediation of subsurface materials generally
involves the stimulation of naturally occurring, or indige-
nous, microorganisms to degrade organic contaminants.
The microorganisms are stimulated by addition of agricul-
tural fertilizers, such as manure; aqueous solutions of nu-
trients, such as ammonia and orthophosphate; and
possibly an oxygen source, such as hydrogen peroxide.
This is typically done by pumping ground water from the
aquifer, treating it to remove contaminants, and adding
nutrients and an oxygen source before reinjecting it into
the aquifer. Water is withdrawn faster than it is reinjected,
creating a pressure sink at the withdrawal point which hy-
draulically contains the contamination and increases the
flow rate of nutrients through the aquifer (Hazardous
Waste Consultant, 1989). In some cases, other environ-
mental parameters such as pH and temperature can be
optimized to stimulate biological activity.
Both aerobic and anaerobic processes are applicable to
the degradation of hazardous materials. Aerobic biode-
gration, which relies on the presence of oxygen, is appli-
cable to remediation of soils contaminated with
nonchlorinated organics, such as fuel oil components,
and some chlorinated materials.
Many chlorinated solvents, such as tetrachloroethene
(PCE), trichloroethene (TCE) and 1,1,1-trichloroethane
(TCA) are resistant to aerobic biodegradation. These
compounds may, however, be degraded under anaerobic
conditions. The degradation of these compounds in-
volves reductive dehalogenation, where chlorine is re-
placed with hydrogen, to form new compounds that may
be more mobile and toxic than the original compound be-
fore being mineralized (Wilson, undated; Ehlke, undated).
Chlorinated alkenes have been mineralized by cometab-
olism, or methane-utilizing bacteria (methanotrophs). In
other contaminated soil systems, some chlorinated com-
pounds can be reductively dehalogenated to produce in-
termediate products that can then be degraded further
aerobically. An example is the anaerobic reductive deha-
logenation of 2,4,5-trichlorophenoxyacetic acid (2,4,5-T)
to 2,4-dichlorophenoxyacetic acid (2,4-D), which can then
be treated aerobically (U.S. EPA, 1990c).
Very often bioremediation is combined with other tech-
nologies, either by design as with pump-and-treat and
in situ bioremediation, or as part of a treatment train,
such as posttreatment following soil flushing or vacuum
extraction.
The benefits of in situ bioremediation include:
Excavation of contaminated materials is not required
It may result in complete degradation of organic con-
taminants to nontoxic byproducts (C02, CH4, HaO)
There are minimal mechanical equipment require-
ments
The limitations of in situ bioremediation include:
There is a potential for partial degradation to equally
toxic, more highly mobile byproducts
It may be difficult to contain volatile organic com-
pounds emitted during remediation
The process is highly sensitive to toxins and environ-
mental conditions
3.3.1 Data Collection Requirements
Data needs that are critical for evaluating whether soil
conditions are favorable for bioremediation are presented
in Table 3-3. Soil conditions may be more easily manipu-
lated to increase the potential for microbial activity for
landfarming, where surface soils may be tilled to distrib-
ute oxygen and nutrients. This may not be possible for
in situ bioremediation due to heterogeneous soils or low
hydraulic conductivity. Soil characteristics specifically im-
portant to the success of in situ treatment are presented
in Table 3-4. Data collection requirements are identified
in Table 3-5.
3.3.2 Evaluation
Several factors to be considered when evaluating a treat-
ment technology for stabilizing a site include (1) how well
the technology will work on the contaminated media, (2)
how long the stabilization process will take using this
technology, (3) the residuals that are produced by the
process, and (4) the cost of the technology as compared
to a technology capable of producing comparable results.
For bioremediation, the first two factors, how well the
43
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technology will work and how long it will take, are most
important in determining the feasibility of this technology
for a particular site. '
A test has been developed for assessing the feasibility of
in situ bioremediation (Hazardous Waste Consultant,
1989). The test consists of a simple scoring system
which weighs the relative influence of three factors:
Nature and biodegradability of contaminants
Permeability, thickness, and location of aquifer
Soil and ground-water geochemistry
Since in situ bioremediation is often implemented in con-
junction with a pump-and-treat system, the ability to ex-
tract and recharge ground water should be evaluated to
determine process feasibility.
Information on degradation rates, or half-lives, of the spe-
cific compounds to be degraded should be obtained in or-
der to evaluate the feasibility of bioremediation. Data on
degradation rates are available in several of the refer-
enced EPA documents (U.S. EPA, 1988a; U.S. EPA,
1990c). Degradation rates can also be determined ex-
perimentally by treatability testing.
Methods for evaluating the feasibility of bioremediation
and for providing data for implementation are discussed
in the following subsections.
3.3.2.1 Treatability Testing. EPA has developed a
bench-scale interim protocol for determining the aerobic
biodegradation potential of organic contaminants in soils.
The protocol is designed to permit evaluation of biode-
gradation rates under different environmental conditions,
e.g., pH, moisture content, and nutrient level. Removal of
contaminants due to abiotic mechanisms (volatilization,
chemical transformation, or adsorption) is also evaluated
under the protocol. The test method is referred to as a
flask test or slurry test. A detailed description of the test
protocol is presented in Determination of Aerobic Degra-
dation Potential for Hazardous Organic Constituents in
Soil-Interim Protocol (U.S. EPA, 1988b).
The EPA protocol does not simulate in situ bioremedia-
tion; however, flask or slurry tests are appropriate for as-
sessing the feasibility of bioremediation, whether in situ
or by landfarming. The tests do not yield information on
the biodegradation rates likely to be observed in situ.
In situ conditions are best simulated at the bench scale
by column tests rather than flask tests. A column test,
however, may not provide a completely representative
model of subsurface conditions if heterogeneous soils
are present. A column test is performed by packing con-
taminated soil into a column and a'pplying water to the
soil. The water may be amended with nutrients, an oxy-
Table 3-3. Critical Environmental Factors for Microbial Activity
Environmental Factor
Purpose/Optimum Levels
Available soil water
Degradation can be limited by insufficient or excess
moisture 25 to 85% of water holding capacity; -0.01 MPa
Oxygen
Aerobic metabolism: greater than 0.2 mg/L dissolved
oxygen, minimum air-filled pore space of 10%; anaerobic
metabolism: Oa concentrations less than 1%
Rodox potential
Aerobes and facultative anaerobes: greater than 50
millivolts; anaerobes: less than 50 millivolts
PH
Nutrients
5.5 - 8.5
Sufficient nitrogen, phosphorus, and other nutrients so as
not to limit microbia] growth (suggested C:N:P weight ratio
of 120:10:1, limit!ngrratio 300:15:1)
Temperature
15-45ฐC
Adapted from U.S. EPA, 1988a; U.S. EPA, 1990c.
44
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Table 3-4. Site and Soil Characteristics for In Situ
Treatment
Site location/topography and slope
Soil type, and extent
Soil profile properties
Boundary characteristics
Depth
Texture*
Amount and type of coarse fragments
Structure*
Color
Degree of mottling
Bulk density*
Clay content
Type of clay
Cation exchange capacity*
Organic matter content*
PH*
Eh*
Aeration status*
Hydraulic properties and conditions
Soil water characteristic curve
Field capacity/permanent wilting point
Water holding capacity*
Permeability* (under saturated and a range of
unsaturated conditions)
Infiltration rates*
Depth to impermeable layer or bedrock
Depth to ground water,* including seasonal variations
Flooding frequency
Runoff potential*
Geological and hydrogeological factors
Subsurface geological features
Ground-water flow patterns and characteristics
Meteorological and climatological data
Wind velocity and direction
Temperature
Precipitation
Water budget
* Factors that may be managed to enhance soil treatment.
Source: Adapted from U.S. EPA, 1984.
gen source, or microbial inocula. Replicate columns are
sacrificed at different times so that soil analyses may be
performed. The soil analyses provide data on contami-
nant concentrations as a function of time. These data are
used to calculate biodegradatfon rates. Testing procedures
for column testing can be found in Stroo et al. (1989).
Anaerobic systems have been tested by using micro-
cosms. Test methods are described in the referenced lit-
erature (Wilson, undated; Ehlke et al., undated; Gibson
et al., undated).
3.3.2.2 Mathematical Models. Modeling, when inte-
grated with site characterization and treatability study re-
sults, can provide information on the rates and extent of
treatment that may be expected at the field scale under
varying conditions and on degradation and partitioning
processes within a system. Several comprehensive
mathematical models are available for evaluating behav-
ior of an environmental system. The Regulatory and In-
vestigative Treatment Zone Model (RITZ Model)
developed at the U.S. EPA Robert S. Kerr Environmental
Research Laboratory has been used to describe the fate
and behavior of organic constituents in a contaminated
soil system (U.S. EPA, 1988c). The Vadose Zone Inter-
active Processes (VIP) model simulates dynamic behav-
ior of organic constituents in unsaturated soil systems
under variable conditions (Stevens et al., 1988c).
3.3.2.3 Field Operation. In some cases, it may be bene-
ficial to proceed with implementation of a bioremediation
system and evaluate the system during operation. Al-
though this method of evaluation can be risky, it may ex-
pedite the stabilization process and may ultimately
reduce costs. This method should be considered for non-
haiogenated hydrocarbon-contaminated soils (such as
fuel oil spills) with little or no inhibitory constituents, such
as heavy metals. For an in situ system, site hydrology
and soil and water chemistry should be known to be fa-
vorable for biological activity.
Evaluation of such a system can be accomplished by
monitoring components such as total petroleum hydro-
carbons (TPH), TOC, and any other representative com-
ponents in the soil and ground water prior to and during
remediation. The contaminants of concern must always
be monitored.
It is also recommended that a control area be established
upgradient of contamination. This should be done in or-
der to compare natural levels of biological activity to the
enhanced biodegradation provided by nutrient and oxy-
gen additions (U.S. EPA, 1990c).
3.3.3 Engineering Considerations for
Implementation
There are a number of factors that should be considered
for the design and implementation of a bioremediation
system. Although these factors generally pertain to any
system, the mechanism by which they are implemented
depends greatly on the type of system, i.e., landfarming,
45
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Table 3-5. Data Requirements for Bioremediation
Data Description
Purpose(s)
Source(s)/Method(s)
Organic contaminant
characterization:
VOCs/SVOCs/PAHs/
PCBs/PestlcIdes
Metals
PH
Type and concentration of
materials in spil matrix
necessary to [evaluate potential
for degradation; determine
potential toxicants, potential
need for mutant bacteria, need
for ancillary VOCs treatment
Needed to determine potential
for microbial inhibition
i
i
Needed to determine whether
adjustment is necessary for
treatment
Organics analyses
U.S. EPA SW-846 (1986)
Metals analyses
U.S. EPA SW-846 (1986)
Soil pH
U.S. EPA SW-846 (1986)
Nitrogen/Phosphorus
Needed to determine whether
supplemental nutrients are
necessary i
Soil analyses for
N03/NH3/P04
Standard methods for analysis
of water and wastewater
Microbial
characterization
Soil permeability
Soil water content
Seasonal temperature
fluctuations
Provides information on micro-
organisms present in the soil
Necessary f of in situ
treatment j
Needed to determine whether
irrigation or drainage is
necessary for microbial
growth I
Needed to determine length
of season available for
bioremediation
Plate count
Percolation testing,
slug tests, pumping tests
ASTMD2216-71 (1973)
Contact National Weather
Service or obtain values
of average temperature
and temperature ranges for the site
Adapted from U.S. EPA, 1988a; U.S. EPA, 1988d; U.S. EPA; 1990c.
46
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aerobic in situ degradation, or anaerobic in situ degrada-
tion. Soil enhancement methods are described below.
3.3.3.1 Enhancement of Biological Mechanisms.
Since biological activity may be greatly improved by en-
hancing certain soil properties or site conditions, it may
be beneficial to design such enhancements into the sys-
tem. However, treatment intended to enhance soil prop-
erties or site conditions must not be implemented in such
a way that it would severely restrict microbial growth or
biochemical activity, or increase mobility of contaminants
in an in situ system.
Soil Moisture. Soil moisture can be increased using
standard agricultural irrigation practices such as over-
head sprinklers or subirrigation. To remove excess water
or lower the water table to prevent water-logging, drain-
age or well point systems can be used. Soil moisture
control can be combined with a pump-and-treat system
where contaminated ground water is extracted, treated to
remove contamination, and amended with nutrient and
an oxygen source, if desirable, before it is reinfiltrated or
used for irrigation.
Oxygen Control. Generally, supplying sufficient oxygen
for microbial activity is a greater implementation concern
for in situ treatment than for other aerobic methods, such
as landfarming. Air can be supplied in situ by injection of
aerated water through well points. Although this is the
most economical method, the oxygen supply is limited by
the solubility of oxygen in water. The addition of hydro-
gen peroxide to reinjection or irrigation water is a com-
mon method of supplying oxygen. This can be
accomplished with basic chemical feed and mixing equip-
ment. High concentrations of hydrogen peroxide are toxic
to microorganisms; however, the microorganisms can
usually be acclimated. An advantage to using hydrogen
peroxide in an injection well system is that it helps keep
the wells free of heavy biogrowth. An alternative ap-
proach is the addition of nitrate to the system, rather than
an oxygen source. This approach has been found to be ef-
fective and more economical in some studies (U.S. EPA,
1990c). Tilling is used to aerate landfarming systems.
Nutrients. Commercial agricultural fertilizers and nutrient
solutions can be used to supply supplemental nitrogen
and phosphorus. Power implements, tillers, and applica-
tors can be used to apply the nutrients to land-based sys-
tems, or nutrients can be added to treated water from a
pump-and-treat system and applied through reinfiltration
or irrigation. Nutrients are necessary for both aerobic and
anaerobic degradation.
Soil Temperature. Land-based bioremediation must be
planned to take advantage of the warm season in cooler
regions of the country and the cooler season in hot, arid
regions. Temperature can be controlled to limit excess
heat loss and gain by several methods. Vegetation can
be used as an insulator to reduce heat loss and limit frost
penetration during cold weather for in situ Systems. Appli-
cation of mulches can help control heat loss and gain, as
well as loss of moisture. For instance, a black paper or
plastic mulch absorbs radiant energy during the day and
reduces heat loss at night. A sprinkle irrigation system
can help provide frost protection during the winter and
cooling during the summer, increase thermal conductivity
of the soil, and provide moisture.
Addition of Commercial Bacteria. Although bioreme-
diation using indigenous bacteria is desirable, augmenta-
tion of biological systems with specifically acclimated or
mutant commercial bacteria may be desirable under cer-
tain conditions. Addition of commercial bacteria may be
desirable when sufficiently large populations of naturally
occurring bacteria cannot be developed or when the pres-
ence of a specific hazardous organic compound requires
use of a specific microorganism. Microbial inoculants with
a broad range of metabolic capabilities are commercially
available. Before applying these bacteria, however, the
actual need must be confirmed. Often, if ah indigenous
bacteria population cannot be developed, conditions are
not favorable for bioremediation. Application methods are
determined in consultation with the supplying vendor
(U.S. EPA, 1988d).
Creating a Reducing Environment for Anaerobic
Degradation. In order to stimulate anaerobic microbial
activity, a relatively oxygen free, reducing environment is
necessary. Methods of limiting the oxygen concentration
in the soil system and creating a reducing environment in-
clude soil compaction to reduce oxygen,diffusion through
large soil pore spaces, keeping the soil wet, deep mulch-
ing to impair oxygen diffusion to the soil, or a combination
of these techniques. A reducing environment can be fur-
ther stimulated by the addition of a reducing agent, or
electron donor, or by adding excessive arhounts of readily
biodegradable organic matter, which will result in oxygen
depletion. Reducing agents include sulfate and sulfide
compounds, which can be added with water. Other elec-
tron donors include fatty acids; however, depending on
the material to be degraded, these materials may either
accelerate or inhibit bioremediation (Ehlke, undated, and
Gibson et al., undated). The addition of methane in an air
stream is beneficial in stimulating methanotrophs, or the
methane-utilizing microorganisms.
3.3.3.2 Ancillary Equipment. For in situ treatment, de-
sign of a pump-and-treat system may be required (see
Chapter 4). It is necessary to determine placement of the
extraction wells, the rate of extraction, the treatment proc-
esses necessary to treat water to the desired quality and
remove inhibitory substances, and the disposition of ex-
cess water. The portion of the treated ground water to be
infiltrated or reinjected must be determined. The point of
discharge and the discharge criteria must be determined
for any portion of water not returned to the system.
47
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In the event that volatilization of organics is occurring
during remediation, collection and treatment of the volk-
tile organics should be considered. Use of an inflatable
plastic dome erected over a contaminated site has been
used to contain volatile emissions. A portable green-
house with an air collection system could be used as
well. The volatile emissions can be treated with a vapdr-
phase activated carbon system or catalytic oxidation unit.
An air discharge permit may be required depending on
the particular state regulations. '
3.4 VACUUM EXTRACTION
Vacuum extraction, also known as soil vapor extraction
and forced air venting, consists of the removal of con-
taminants by drawing clean air through a zone of con-
taminated soil. Contaminants desorb from the soil and
are carried away with the exhausted air. Continued flush-
ing with clean air can result in a significant decrease in
the concentration of volatile compounds in soil.
The basic components of a vacuum extraction system
are extraction wells and a blower. In most cases, mois-
ture separation and off-gas treatment will also be re-
quired in order to meet air discharge requirements.
Recharge wells, an impermeable cover, conditioning df
recharge air, flow control and measurement instrumenta-
tion, vapor concentration monitoring, and other enhance-
ments are also frequently added in order to improve
system performance and flexibility.
The physical basis of the technique rests on the tendency
of many volatile compounds to diffuse from the soil matrix
to the air in pore spaces as result of the concentration dif-
ference between the soil and the clean air that is intro-
duced. Once the contaminants have become entrained in
the soil atmosphere, they are carried out of the sc-il
through the circulation of fresh air. The effectiveness pf
vacuum extraction is therefore related to those properties
that determine the extent to which contaminants diffuse
into the soil atmosphere and the effort required to remove
the contaminant-laden air from the soil.
Vacuum extraction is most likely to be successful at sites
where highly volatile contaminants are present in homo-
geneous soils of high permeability and porosity. Trie
benefits of using vacuum extraction include:
Implementation can be conducted in situ and requires
relatively little disturbance to existing facilities or
operations
The process reduces contaminant concentration and
mobility at the treated area
Implementation can be flexible, allowing for adaptation
to changing site conditions or additional analytical and
subsurface data
In situ installation and operation requires little handling
of contaminated materials, limiting the risk of exposure
to workers and the public !
Vacuum extraction has few secondary impacts; only
ambient air need be introduced into contaminated
soils, and potentially harmful reagents are not
required
Vacuum extraction is only applicable to volatile contami-
nants; alternative treatment will be required for nonvola-
tile compounds. Vacuum extraction will be costly and
may require prohibitive operation times to achieve
cleanup at sites where soil is heterogeneous or has a low
air permeability. Other disadvantages of using vacuum
extraction include:
Soils must be permeable and fairly homogeneous for
the technique to be efficient; impermeable lenses may
adversely affect the results of the process
Cleanup to low levels can be difficult and require
lengthy remediation time with the potential for greater
than anticipated operation and maintenance costs,
particularly in heterogeneous soils
Verification of complete cleanup effectiveness can be
difficult, particularly in heterogeneous soils
Off-gas treatment may produce RCRA-regulated
wastes, which will require special handling and dis-
posal practices
3.4.1 Data Collection Requirements
Several chemical and physical characteristics of the soil
to be treated should be determined or estimated in order
to assess the applicability of vacuum extraction to a par-
ticular release. These characteristics can be grouped into
physical factors which control the rate of air flow through
the contaminated portion of the soil and chemical charac-
teristics which determine the amount of contaminant that
partitions from the soil to the air. There are environmental
considerations which must be evaluated in the design of
a vacuum extraction system. The specific data that
should be collected and evaluated in assessing the feasi-
bility and developing the design of the technique are pre-
sented in Table 3-6.
3.4.2 Evaluation
The feasibility of vacuum extraction at a particular site
can usually be evaluated with data that can be easily es-
timated or obtained from a limited boring program, which
is frequently conducted as part of the initial stages of a
RCRA facility assessment. Vacuum extraction will typi-
cally be applicable if the majority of the following condi-
tions are met (U.S. EPA, 1990a):
Dominant contaminant form is vapor phase
Soil temperature is greater than 20ฐC (in the absence of
external heating, soils seldom reach this temperature)
Soil air conductivity is greater than 10"4 cm/s
Moisture content of soil is less than 10 percent by
volume
48
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Contaminated soil is homogeneous
Surface area of soil matrix is less than 0.1 square me-
ters per gram of soil
Vapor pressure of contaminant is greater than 100 mil-
limeters of mercury
Water solubility of contaminant is less than 100 milli-
grams/liter
Under the following conditions, vacuum extraction will
typically not be feasible (U.S. EPA, 1990a), unless other
remedial actions are not available:
Dominant contaminant phase is solid or is strongly
sorbed to soil
Soil temperature is below 10ฐC (true at depth in many
northern climates)
Soil air conductivity is less than 1O^cm/s
Soil moisture content is greater than 30 percent by vol-
ume or near saturation
Contaminated soil is heterogeneous
Surface area of soil matrix is greater than 1 square
meter per gram of soil
Depth to ground water is less than 1 meter
Vapor pressure of contaminant is less than 10 millime-
ters of mercury
Water solubility of contaminant is greater than 1,000
milligrams/ liter
If site conditions fall 'between these ranges or are ap-
proximately equally split between the lists, vacuum ex-
traction may warrant further evaluation, particularly if
other technologies which are well suited to the situation
cannot be identified. The formulae and discussion pre-
sented by Johnson et al. (1990) and Wilson et al. (1988)
can be used for more detailed evaluation and preliminary
design of vacuum extraction systems, including develop-
ment of information required for rough cost estimates.
For situations that do not allow a technology to be se-
lected on the basis of the screening approach above, ad-
ditional investigations will typically be required for
detailed evaluation.
After it is determined that vacuum extraction may be ap-
plicable to the specific site conditions, additional data will
be required to confirm the feasibility of the technology
and to prepare a final system design. The most important
pieces of data are:
The extent of contamination; ideally, a comprehensive
study requiring an extensive boring and sampling pro-
gram would be possible for use as an interim meas-
ure; however, many sites may be characterized by a
relatively inexpensive soil-gas survey augmented with
chemical data from a limited boring program.
More accurate estimates of soil permeability; initial val-
ues can be estimated from air flow through vapor sam-
pling ports during a soil-gas survey and/or from
laboratory measurement methods. Final design should
include a pump test with one or more observation
wells ("interference test").
The potential effects of ground-water mounding below
the extraction system and required response.
The proximity of receptors.
The proximity of additional potential sources.
The flexibility of the vacuum extraction technology sug-
gests an alternate development path which has been
used at many sites. The process of adaptive or incre-
mental design allows the information gained from pre-
vious phases of remediation to be directly applied to
system improvements. In this type of response, based on
limited data (i.e., a limited boring and sampling program
and a soil-gas survey), a preliminary system design can
be developed. This preliminary system is then imple-
mented, and performance closely monitored during the
initial phases of operation. Based on air flow and con-
taminant recovery data during initial operation, modifica-
tions can be made to enhance system operation. This
process has two major advantages; a treatment system
can be implemented rapidly due to reduced needs for de-
tailed study, and due to modifications a better site-spe-
cific system can be built at lower cost. The greatest
limitation of this approach is that it requires timely agree-
ment between the regulator, the site operator, and the
contractor performing the work.
3.4.3 Engineering Considerations for
Implementation
The key elements of a vacuum extraction system are
shown in Figure 3-2. The system consists of extraction
well(s) and a vacuum pump. The extraction well is in-
stalled near the center of the contaminated area, and a
vacuum is applied to the well. The resulting flow of air
strips contaminants from the soil, which are then dis-
charged to the atmosphere after treatment, if required.
Extraction wells are typically constructed in a fashion
similar to ground-water monitoring wells, with the excep-
tion that the well is screened in the unsaturated zone
(see Rgure 3-3). If multiple extraction wells are used,
they may be pumped through a manifold connected to a
single vacuum pump or to multiple pumps. Piping and
manifolds should be sloped to drain condensate to a cen-
tral storage tank or other discharge point. In cooler cli-
mates, piping should be buried below the frost level or
heat-traced to prevent blockage of the pipe by frozen
condensate.
In most situations, control of vapor emissions will be re-
quired prior to discharge of collected gases. The primary
means of treating off-gases are catalytic incineration for
relatively high contaminant concentrations, and vapor-
49
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Table 3-6. Data Requirements for Vacuum Extraction
Data Description
Purpose(s)
Source(s)/Method(s)
Physical Characteristics
Permeability
Heterogeneity
Depth of air
permeable zone
Depth to water
table, if
different from
above
Porosity
Moisture
Content
Particle size
distribution/
soil structure
and surface area
Soil temperature
Chemical Characteristics
Extent of
contamination
Determine whether sufficient
airflow rates can be
established; estimate system
operating parameters for
design (permeability will
change with the moisture
content of th!e soil)
Characterize; extent and
frequency of impermeable
zones which will be more
resistant to cleanup
Determine depth of zone
that will be cleansed by
extraction process
Determine need for
i
lowering water table
if contamination is below
height of anticipated
water table rise
Determine ajr velocity
in soil pore space
Determine need for and
effectiveness of moisture
control measures to
maintain adequate permeability
Determine adsorptive
capacity of soil
Will affect volatility
of contaminant
Determine area to be
remediated; determine
baseline affected area
prior to treatment
Laboratory: Evans (1985)
Field: Johnson etal. (1990)
Soil boring program
Soil boring program
Monitoring wells
Vomocil(1965)
ASTMD2216
ASTM D 422
ASTMD1140
Soil-gas survey
Soil sampling program
50
-------
Table 3-6 (Continued).
Data Description
Purpose(s)
Source(s)/Method(s)
Degree of
contamination
Vapor pressure/
Henry's law
constant
octanol/water
partition
coefficient
Organic fraction
of soil
Diffusivity
Determine whether sufficient
concentrations are present;
determine baseline concentra-
tions prior to treatment
Determine ability of method
to strip contaminant from
soil
Estimate retention of
contaminant in soil at low
concentrations
Estimate diffusion rate
through low permeability
heterogeneities
Soil sampling program
Literature study. Sources
include Johnson et al.
(1990), U.S. EPA (1989a),
Verschueren (1983).
SW-846 method 9060
Fuller etal. (1966)
Environmental Setting
Proximity of
other
contaminant
sources
Determine whether vacuum
extraction system can
cause migration of
additional contamination
to the site
Site survey and historical
records; soil-gas survey;
soil sampling program
phase carbon adsorption for lower concentrations. Carb-
on may be regenerated (on site or off site) or disposed of
after a single use. In some situations, carbon disposal
will be regulated under RCRA land disposal regulations.
Industrial facilities can frequently incorporate the waste
air stream into other site processes, i.e., as air supply to
an existing incinerator or as part of the air stream to an
existing treatment unit. Moisture control can also be an
important consideration in off-gas treatment, particularly
if the treatment option involves carbon adsorption. Con-
densate from moisture removal may also be a RCRA-
regulated waste.
In order to increase control of subsurface air flow and ef-
ficiently remediate larger areas, vacuum extraction sys-
tems are frequently augmented with a system of air
injection wells (usually at the perimeter of the contami-
nated area). Air injection wells can increase the soil air
flow rate and the area through which clean air will flow.
To further increase removal efficiencies, the recharge air
can be warmed by heat exchangers (electrical or steam)
or through addition of steam to the recharge air. Addition
of large quantities of steam should be used with caution;
however, due to the potential for steam to condense in
the soil and facilitate migration of the contaminants into
the ground water.
The effectiveness of vacuum extraction can sometimes
be increased by the installation of an air-tight cover over
the site. The effect of the cover is to prevent surface air
flow near recovery wells from providing a disproportion-
ate amount of total air flow in the well, which is of major
importance for shallow extraction systems. Surface cov-
ers are typically sloped to provide surface water drainage
and to limit the migration of contaminants to the ground
water through infiltration of rain water.
As noted at one site in Utah, vacuum extraction may be
combined with bioremediation to accelerate the stabiliza-
51
-------
tion process. This process, which involves the addition!of
moisture and nutrients to the soil during venting, can be
particularly useful for remediation of low volatility con-
taminants and can reduce contaminant concentrations in
exhaust gases (Dupont et al., 1991). ]
For sites where ground water is at or near the zone of soil
contamination, it will often be necessary to conduct
ground-water pumping concurrently with vacuum extrac-
tion. Ground-water pumping will counteract the water ta-
ble rise caused by reduced air pressure near extraction
wells. Lowering the ground water may also expose addi-
tional contaminated soil and increase the degree of treat-
ment by vacuum extraction. The technical consideratiohs
relevant to ground-water pumping are addressed in Sec-
tion 4.1. i
Ground-water contamination may be present at many re-
lease sites where vacuum extraction is applicable. In
such situations, concurrent ground-water cleanup may be
necessary due to the potential for adverse effect on re-
ceptors, the need to remediate or contain the ground-
water plume, or the potential for contaminated ground
water to recontaminate the unsaturated zone.
A variety of ground-water cleanup technologies can be
applied in conjunction with vacuum extraction, including
pump-and-treat, bioremediation, and chemical extractipn,
which are described in other sections of this document.
Installations can be constructed to allow concurrent u|se
of a single well for both vacuum extraction and ground-
water pumping. ,
Soil heterogeneity typically reduces the effectiveness of
vacuum extraction because of the formation of preferen-
tial flow paths during treatment. In such cases, air flows
predominantly through the permeable soils, and the less
permeable zones are left relatively untreated. The detri-
mental effects of some heterogeneities or anisotropijes
can be avoided, however, by inducing airflow across loyv-
permeability layers. This technique is described in the
Superfund Innovative Technology Evaluation of soil vapor
extraction at Groveland, Massachusetts (U.S. EpA,
1989b). i
The performance of a vacuum extraction system is moni-
tored by both internal system parameters and external
conditions. Internal parameters include strength of vac-
uum applied, air flow rate, and contaminant concentra-
tion. These parameters can be monitored at each
individual extraction well or at the blower. External moni-
toring includes vapor monitoring wells, which are used ito
determine vapor flow paths from pressure distributions
and to collect vapor samples for chemical analysis; and
soil borings, which may be required to determine the ac-
tual residual concentration of contaminants in the soil.
Based on monitoring data, system parameters such as
flow rate and inlet gas temperature may be altered to op-
timize cleanup efficiency.
In order to control the system operation, valves are typi-
cally installed at each wellhead, these valves are then
used to restrict flow from less contaminated areas and fo-
cus cleanup on the most contaminated soils. In addition,
changing flow rates or selectively cycling wells on and off
will alter the flow paths of air through the soil, preventing
the formation of stagnant zones between wells where air
flow velocities would otherwise be very small. Recharge
wells can also be strategically placed within the contami-
nated area to counteract stagnation of air flow.
Several strategies have been used to determine when a
system has satisfactorily cleaned a site and may be shut
Vapor Treatment
Air/Waler Separator
Extraction Well
Inlet Well
II VVOH 1
Impermeable Cover
Vadose Zone
: Water Table*
Figure 3-2. Schematic of a Vacuum Extraction System
(Adapted from U.S. EPA, 1990d).
Rise
r
L%"V"%"V'A^'^'H"S"S"S"
2- to 4" PVC Casing
Stoned PVC
Soil -ป
10" Auger Hole
r -
fj
>J
>
'j
,\
v
-------
down. Since cleanup goals are typically expressed as
permissible soil concentrations, the most direct and usu-
ally most reliable method is direct soil sampling. Vapor
monitoring wells and pumped air sampling may also be
useful in evaluating system performance. Comparisons of
the amount of contaminant recovered with estimates of
the contaminant mass in the soil have not proved to be
reliable indicators of complete cleanup (U.S. EPA,
1989c).
The vacuum extraction equipment requires a power
source, and may require equipment shelter, depending
upon the anticipated duration of the remediation. For all
components of the system, the potential to accumulate
hazardous vapors should be considered. Equipment
shelters should have adequate ventilation to ensure per-
sonnel safety. Systems for treatment of flammable va-
pors should use explosion-proof equipment as required.
3.5 REFERENCES
American Public Health Association (APHA). 1971. Stand-
ard Methods for the Examination of Water and Wastewater.
American Public Health Association, New York.
American Society of Testing and Materials (ASTM).
1973. Annual Book of ASTM Standards, Part II. Philadel-
phia, PA.
Conner, Jesse R. 1990. Chemical Fixation and Solidifica-
tion of Hazardous Wastes. Van Nostrand Reinhold, New
York, NY.
Dupont, R.R., W.J. Doucette, and R.E. Hinchee. 1991.
Assessment of In Situ Bioremediation Potential and the
Application of Bioventing at a Fuel-Contaminated Site.
Presented in Proceedings of the In Situ and On-Site
Bioremclamation International Symposium - San Diego,
3/19-3/21.
Ehlke, T.A., et al. Undated. Biotransformation of cis-1,2-
Dichloroethylene in Aquifer Material from Picatinny Arse-
nal, Morris County, New Jersey.
Evans, D.D. 1965. Gas Movement. In: Methods of Soil
Analysis: Physical and Mineralogical Properties, Includ-
ing Statistics of Measurement and Sampling (C.A. Black,
editor-in-chief). American Society of Agronomy.
Fuller, E.N., P.O. Schettler, and J.C. Giddigs. 1966. A
New Method for Prediction, of Binary Gas-Phase Diffu-
sion Coefficients. Indus. Engr. Chem. 58,19-27.
Gibson, S.A. and G. W. Sewell. Undated. Stimulation of
Reductive Dechlorination of Tetrachloroethene (PCE) in
Anaerobic Aquifer Microcosms by Addition of Short Chain
Organic Fatty Acids, Alcohols, Sugars, or Aromatic Com-
pounds. Robert S. Kerr Environmental Research Labora-
tory, Ada, OK.
Hazardous Waste Consultant. 1989. July/August.
Johnson, P.C., C.C. Stanley, M.W. Kemblowski, D.L. By-
ers, and J.D. Colthart. 1990. A Practical Approach to the
Design, Operation, and Monitoring of In Situ Soil Venting
Systems. Groundwater Monitoring Review, Vol. 10,
No. 2.
Sowers, C.B. and G.F. Sowers. 1970. Introductory Soil
Mechanics and Foundations, 3rd ed. The MacMillan Co.,
London.
Stevens, O.K., W.J. Grenney, and Z. Yan. 1988. User's
Manual: Vadose Zone Interactive Processes Model. De-
partment of Civil and Environmental Engineering, Utah
State University, Logan, UT.
Stroo, H. et al. 1989. Development of an In Situ Bioreme-
diation System for a Creosote-Contaminated Site. Inter-
national Conference on Physiochemical and Biological
Detoxification of Hazardous Wastes, HMCRI.
Thomas, J.M. and C.H. Ward. 1989. In Situ Bioremedia-
tion of Organic Contaminants in the Subsurface. Environ-
mental Science and Technology, Vol. 23, No. 7,
pp. 760-766.
U.S. EPA. 1991. Draft Quick Reference Fact Sheet: Im-
mobilization as Treatment. Office of Solid Waste and
Emergency Response. Publication 9380.3-07F2.
U.S. EPA. 1990a. Assessing UST Corrective Action
Technologies: Site Assessment and Selection of Unsatu-
rated Zone Treatment Technologies. EPA/600/2-
90/011. Risk Reduction Engineering Laboratory,
Cincinnati, OH.
U.S. EPA. 1990b. Subsurface Contamination Reference
Guide. EPA/540/2-90/011. Office of Remedial Response,
Washington, D.C.
U.S. EPA. 1990c. Handbook on In Situ Treatment of Haz-
ardous Waste-Contaminated Soils. EPA/540/2-90/002.
Risk Reduction Engineering Laboratory, Cincinnati, OH.
U.S. EPA. 1990d. Ground-Water Handbook. Volume 1:
Ground Water and Contamination. EPA/625/6-90/016a.
Office of Research and Development, Cincinnati, OH.
U.S. EPA. 1989a. Stabilization/Solidification of CERCLA
and RCRA Wastes. EPA/625/6-89/022.
U.S. EPA. 1989b. Technology Evaluation Report: SITE
Program Demonstration Test, Terra Vac In Situ Vacuum
Extraction System, Groveland, Massachusetts. Volume I.
EPA/540/5-89/003a.
U.S. EPA. 1989c. State of Technology Review Soil Vapor
Extraction Systems. EPA/600/2-89/024.
U.S. EPA. 1988a. Bioremediation of Contaminated Sur-
face Soils. EPA/600/2-89-073.
U.S. EPA. 1988b. Determination of Aerobic Degradation
Potential for Hazardous Organic Constituents in Soil - In-
terim Protocol. Risk Reduction < Engineering Laboratory,
Cincinnati, OH.
U.S. EPA. 1988c. Interactive Simulation of the Fate of
Hazardous Chemicals during Land Treatment of Oily
Wastes: RITZ User's Guide. EPA/600/8-88-001. Robert
53
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S. Kerr Environmental Research Laboratory, U.S. EPA,
Ada, OK.
U.S. EPA. 1988d. Technology Screening Guide for Treat-
ment of CERCLA Soils and Sludges. EPA/540/2-88-004.
NTIS. :
U.S. EPA 1986a. Handbook for Stabilization/Solidification
of Hazardous Waste. EPA/540/2-86/001. [
U.S. EPA. 1986b. Prohibition of the Disposal of Bulk Liq-
uid Hazardous Wastes in Landfills - Statutory Interpretive
Guidance. EPA/530-SW-106. Office of Solid Waste and
Emergency Response. :
U.S. EPA. 1986c. Test Methods for Evaluating Solid
Waste. SW-846. :
U.S. EPA. 1986d. Systems to Accelerate In Situ Stabili-
zation of Waste Deposits. EPA/540/2-86/002. Hazardous
Waste Engineering Research Laboratory, Cincin-
nati, OH. |
U.S. EPA. 1984. Review of In Place Treatment Technolo-
gies for Contaminated Surface Soils. Vol. 2: Background
Information for In Situ Treatment. EPA/540/2-84-0032.
U.S. EPA. 1979. Manual of Methods for Chemical Analy-
sis of Water and Wastes. EPA/600/4-79/020.
Verschueren. K. 1983. Handbook of Environmental Data
on Organic Chemicals. 2nd edition. Van Nostrand Rein-
hold, New York, NY.
Vomocil, James A. 1965. Porosity. In: Methods of Soil
Analysis: Physical and Mineralogical Properties, Includ-
ing Statistics of Measurement and Sampling (C.A. Black,
editor-in-chief). American Society of Agronomy.
Wilson, B.H. Undated. Biotransformation of Chlorinated
Hydrocarbons and Alkylbenzene in Aquifer Material from
the Picatinny Arsenal, NJ. Environment and Groundwa-
ter Institute, University of Oklahoma.
Wilson, David J., Ann N. Clarke, and James H. Clarke.
1988. Soil Cleanup by In Situ Aeration. 1. Mathematical
Modeling. Separation Science and Technology, Vol. 23,
No. 10.
54
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CHAPTER FOUR
Water Treatment Technologies
This section presents a thorough discussion of ground-
water extraction required for the pump-and-treat method
of ground-water remediation, and a screening matrix of
some of the more commonly used, easily implemented
treatment technologies. Due to the wealth of information
available on the conventional treatment technologies tra-
ditionally used for industrial water or wastewater treat-
ment, a thorough discussion was not deemed necessary.
4.1 PUMP-AND-TREAT
Pump-and-treat remediation technology refers to the ex-
traction of contaminated ground water and subsequent
treatment of the extracted ground water at the surface.
Extraction of contaminated ground water is accomplished
through the use, of extraction wells or drains which are
completed at specified locations and depths to optimize
contaminant recovery. Determination of the location and
depth of extraction wells or drains requires prior deline-
ation of the contaminant plume and knowledge of the
aquifer properties such as hydraulic conductivity (K), stor-
age, stratigraphy, and depth to bedrock. Treated water
may be discharged to injection wells or recharge basins
that are positioned to enhance contaminant recovery by
flushing contaminants toward extraction wells. Treated
water may also be discharged directly to surface water
(NPDES permit is normally required). Pump-and-treat
with reinfiltration or reinjection can be combined with in
situ soil treatment systems such as flushing and/or biore-
mediation to recover contaminants in both the vadose
zone and ground water. Pump-and-treat systems may
also be used with containment technologies such as
slurry cutoff walls to limit the amount of clean water flow-
ing to the extraction wells, thus reducing the volume of
water to be treated.
Pump-and-treat systems are generally considered at
sites where significant levels of ground-water contamina-
tion exist. In order for this technology to be effective, it
must be possible for contaminants to readily flow to ex-
traction wells, thus the subsurface must have sufficient
hydraulic conductivity (i.e., K>10'5 cm/s) and the chemi-
cals must be transportable by the ground water (i.e., not
strongly sorbed to soils). This technology is often desir-
able from an implementation standpoint because it can
be designed to be versatile in operation and often can be
modified or augmented during operation to enhance per-
formance as data are collected. For example, if it is de-
termined during operation that a system is not as effec-
tive in a certain contaminated area as originally expected,
additional extraction wells could be installed to enhance
recovery. However, it is often not as easy to modify treat-
ment system designs during operation; consequently, to-
tal system flowrates should be known with certainty
during design.
The main limitation of the pump-and-treat technology is
the long time that may be required to achieve an accept-
able level of cleanup (U.S. EPA, 1990a). The following'
factors contribute to this problem and therefore may limit
the applicability of the pump-and-treat technology to cer-
tain sites: (1) the presence of chemicals with relatively
low mobility (e.g., PCBs, dioxin); (2) aquifers with low hy-
draulic conductivities (i.e., <10-7cm/sec); (3) highly het-
erogeneous hydrogeologic settings (e.g., highly stratified
aquifers with multiple layers of coarse and fine textured
material); and (4) the presence of a spatially discontinu-
ous or inaccessible dense nonaqueous phase liquid
(DNAPL).
NAPLs in general can complicate remediation because
they can become trapped in pore spaces by capillary
forces and are not readily pumped out. The residual satu-
ration in the pore spaces can be a significant source of
contamination, and pump-and-treat removal is therefore
limited by how fast the NAPL components can dissolve.
For highly heterogeneous conditions pumped ground
water will sweep through zones of higher hydraulic con-
ductivity, recovering contaminants from those zones, but
will be less effective in removing contaminants from low
conductivity zones. Movement of contaminants from
these zones is a much slower process and could in-
crease cleanup times significantly; thus, pump-and-treat
technologies may work in heterogeneous media, but
cleanup times will be longer than in homogenous mate-
rial. For higher sorbing compounds, cleanup times will be
longer because more pore volumes must be removed in
order for contaminants to be flushed away.
All of the above items contribute to a phenomenon called
"tailing." Tailing is the asymptotic decrease of contami-
nant concentration in water that is removed in the
cleanup process and is often observed during implemen-
tation of this technology (U.S. EPA, 1990a).
55
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The surface treatment of extracted ground water will vary
depending on the contaminants present. Treatment tech-
nologies to be used may include air stripping, activated
carbon adsorption, and biological treatment. In some
cases, treated ground water may be amended with nutri-
ents and oxygen and reinjected into the subsurface to aid
in stimulating biodegradation processes.
Variations in the operation of the extraction system can
affect the recovery of the contaminants. The two basic
modes of extraction are continuous and pulsed pumpirjg.
Continuous pumping refers to uninterrupted extraction [Of
ground water to create a capture zone around the plume
of contamination, thereby controlling migration while the
contaminants are flushed from the aquifer. The pumping
rate is limited by the yield of the aquifer. Pulsed pumping
involves regular or periodic cessation of pumping activi-
ties to enhance the extraction of ground-water contami-
nants. Pulsed pumping may be necessary or more cost
effective in cases where extraction wells cannot sustain
even moderate yields (e.g., in bedrock and unconsoli-
dated deposits of low permeability), where desorptilan
and/or dissolution of contaminants in the subsurface is
relatively slow, or where hydraulic conductivity heteroge-
neity is high. In the latter two cases, pulsed pumping
would result in periodic low ground-water velocities, al-
lowing contaminants to build up equilibrium concentra-
tions locally and allow more efficient removal of
contaminants. In general, pulsed pumping may be appro-
priate for (1) low-yield consolidated and unconsolidatled
deposits; (2) low-mobility contaminants; (3) heterogene-
ous formations consisting of alternating high- and lovv-
permeabilrty layers and containing contaminants; and
(4) hydrogeological settings containing low to moderately
soluble residual NAPLs. The instantaneous pumping rate
may exceed the yield of the aquifer, but the long-term
rate of withdrawal is limited by it.
Treatment system operation regimes include continu-
ous flow, intermittent continuous flow, or batch opera-
tion. Selection of the treatment system operation is
dependent upon both the ground-water extractipn
system operation as well as treatment technology
limitations and economics. ,
Treated water may be discharged to the aquifer either to
enhance contaminant containment or recovery or simply
for disposal. Reinjection refers to injection of treated
ground water into the subsurface via wells. Recharge re-
fers to the use of trenches, basins, or infiltration galleries
to reintroduce the treated water near the ground surface.
Recharged ground water can be used for in situ soil tre|at-
ment by flushing soils in the unsaturated zone. Recharge
or reinjection also may be used to introduce nutrients and
biological organisms to enhance biodegradation proc-
esses in the saturated zone. This has proven effective
particularly in the case of most light nonaqueous phase
liquids (LNAPLs) that are derivatives of petroleum
products.
4.1.1 Data Collection Requirements
The data required to make decisions regarding pump-
and-treat technologies are shown in Table 4-1. The first
four items are minimum data requirements for determin-
ing applicability of pump-and-treat technology. The re-
maining items must be considered for implementation
and are required to design a system and evaluate system
effectiveness.
4.1.2 Evaluation
The data evaluation for the ground-water extraction com-
ponent of a pump-and-treat system generally involves a
quantitative analysis of ground-water flow and contami-
nant transport. The parameters and conditions listed in
Table 4-1 are used as input into analytical equations or
numerical models to quantify natural flow conditions and
the impacts of pump-and-treat systems. In the initial
stages of the decision process, when it must be deter-
mined whether pump-and-treat is applicable, simplified
methods can be employed. Simplified methods are avail-
able for evaluating the following (U.S. EPA, 1985a):
(1) well hydraulics, (2) drain hydraulics, (3) ground-water
mounding, (4) seepage/infiltration, (5) effects of multiple
wells (superposition), and (6) contaminant transport.
These methods can be used to determine if adequate re-
covery or containment can be attained. For example,
simplified equations for well hydraulics can be used to
estimate flow rates and associated zones of capture for a
well or series of wells within a contaminated area. Given
these flow rates, simple equations for contaminant trans-
port or pore volume removals can be used to estimate
cleanup time. If the rate, capture zones, and cleanup
times appear feasible, then it is likely that the technology
is applicable. Once this has been determined, a more de-
tailed analysis can be undertaken to evaluate system ef-
fectiveness using these equations or more sophisticated
tools like semi-analytical or numerical models. These are
also useful tools for designing systems with regard to
system configuration, flow rates, operational considera-
tions, and cleanup times. If time and funding are avail-
able, numerical modeling should be employed in the
design process to lead to more accurate and confident
decisions regarding implementation of this technology.
4.1.3 Engineering Considerations for
Implementation
Limitation of the pump-and-treat technology in terms of
complete and timely restoration of water quality to "clean"
status has been the subject of much recent literature. An
analysis of operational experience at a number of active
remediation sites is presented in U.S. EPA, 1989. This
document concludes that contaminant migration control
is more easily achieved than cleanup, that stratigraphi-
cally simple sites are easier to clean up than complex
ones, that predesign data collection and operational
monitoring are typically inadequate, and that the system
56
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Table 4-1. Data Requirements for Pump-and-Treat Systems
Data Description
Purpose(s)
Source(s)/Method(s)
Hydraulic conductivities
and storativities of
subsurface materials
Contaminant
concentrations and
areal extent
Contaminant/soil properties
(density, aqueous solubility
octanol water, carbon
partitioning coefficient,
organic carbon content
of soil)
Determine feasibility
of extracting ground
water; determine
applicability of pump-
and-treat technology
Determine seriousness of
problem; determine
applicability and
evaluate effectiveness
Determine mobility
properties, existence
of NAPL; determine
applicability of
pump-and-treat
Pumping tests, slug tests,
laboratory permeability
tests
Water quality data
Literature (Wyman
etal., 1982);
laboratory tests
Types, thicknesses,
and extent of saturated
and unsaturated sub-
surface materials
Depth to aquifer/
water table
Ground-water flow
direction and
vertical/horizontal
gradients
Seasonal changes
in ground-water
elevation
NAPL density/
viscosity/solubility
Ground-water/surface
water connection
Develop conceptual design;
determine applicability/
considerations for
implementation
Select appropriate
extraction system type;
considerations for
implementation
To determine proper well
locations/spacing
considerations for
implementation
Used in locating wells
and screened intervals;
considerations for
implementation
Predict vertical dis-
tribution of contamina-
tion; considerations
for implementation and
evaluating effectiveness
Determine impacts of
surface water
Hydrogeologic maps,
surficial.geology
maps/reports, boring
logs, geophysics
Hydrogeologic maps,
observation wells,
boring logs,
piezometers
Water level data
Long-term water level
monitoring
Literature
Seepage measurements
stream gaging
57
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Table 4-1 (Continued)
Data Description
Purpose(s)
Source(s)/Method(s)
Locations, screen/
open Interval depths,
and pumping rates of
wells influenced
by site
Precipitation/
recharge
Determine impacts/
interference;
considerations for
implementation and
evaluate effectiveness
Used in calculating
water balance;
considerations for
implementation and
evaluate effectiveness
Well inventory,
pumpage records
NOAA reports, local
weather bureaus
may have to be changed after a period of operation to
compensate either for changing conditions or for original
design misconceptions. !
In designing and implementing pump-and-treat system^,
consideration must be given to the goals and objectives
of the system. Oftentimes these are defined by the gov-
erning agency and are based on risk and endangerment
assessments. In other cases, a problem might be appar-
ent but the risk to the environment may not have been
ascertained. In any case, the severity of the problefn
must be considered in determining how to design and im-
plement the system. If the goal of the system is to clean
the site as quickly as possible, then a more active pump-
and-treat system may be designed focusing on aggres-
sive removal of contaminants. An example of aggressive
removal v/ould be a pump-and-treat system installed
within a totally encapsulating slurry wall. Such a systein
has been installed at the Gilson Road Superfund Site in
New Hampshire (Weston, 1989). If the goal is simply to
prevent offsite migration of contamination, then a more
passive system may be designed such as a system con-
sisting of interceptor wells or trenches.
Site conditions, particularly, site hydrology and contami-
nant conditions, must be well defined in order to properly
evaluate and implement the pump-and-treat technology.
Limited information on contamination and site hydrogeol-
ogy resulted in problems at several of the facilities visited
during the preparation of this document. At one site, a
soil-gas survey was performed to locate wells. It was
found that soil-gas proved to be an excellent technique
for establishing the areal extent of contamination in the
shallow contaminated zone but did not distinguish be-
tween contamination in the vadose zone and ground
water. It also did not detect contamination in areas whefe
the depth to ground water exceeded 400 feet. As a result,
TCE contamination that had migrated off site was not in-
itially detected, and an interim measure that had been
proposed to recover contaminated ground water had to
be abandoned.
At other sites visited where this technology was imple-
mented before sufficient site characterization data had
been collected, it was found that significant changes in
design and operation had to be made during implementa-
tion because adequate recovery of contaminated ground
water did not occur. This resulted in increased costs and
continued offsite migration of contaminants.
Site aspects other than those presented under data re-
quirements that should be considered during design and
implementation include the following:
The location of surface water bodies and other ground-
water bodies with respect to the contaminated aquifer
Cleanup standards or standards of the receiving body
that must be maintained (e.g., SDWA MCLs, drinking
water standards, local classifications of ground water
and surface bodies)
The location of nearby wetlands
The size of the site property and the availability of land
for installing a pump-and-treat system
Past experience in applying this technology in similar
site conditions
There are definite drawbacks to implementing this tech-
nology without sufficient site characterization data. An at-
tempt should be made to obtain enough information to
understand the ground-water flow contaminant migration
processes at the site to prevent the implementation of
costly ineffective measures.
As with any stabilization technology, monitoring is re-
quired to ensure the effectiveness of the system. For the
pump-and-treat technology, this may involve periodic
measurement of the following:
Water levelsto ensure adequate capture zones
58
-------
Water qualitymeasured at monitoring wells, soil-gas
surveys, surface water
Treatment system effluentto ensure adequate con-
taminant removal
The results of monitoring should continuously be used to
evaluate system operations and to possibly make
changes or additions to the system. This is particularly
true for systems that have been implemented with mini-
mal site characterization data for design.
Monitoring results will be required to determine when a
system should be shut off. Postclosure monitoring may
be required to ensure that concentrations do not increase
.after system shut-off. This could occur if ground-water
velocities are too high to remove sorbed contaminants of
NAPL residual saturation or if contaminants existing
above the cone of depression of extraction wells have not
been removed.
The design process should begin with the development
of a conceptual design based on the above considera-
tions. The next step would then be the design of the
physical aspects of the system assuming that the re-
quired data to define subsurface conditions have been
collected.
The first design aspects that must be determined for a
pump-and-treat system are horizontal and vertical spac-
ing of wells, pumping rates, and possibly cleanup times.
Once these have been determined, components such as
wells, pumps, piping, and treatment facilities can be de-
signed. The design of the latter features are based on
standard practices.
Analytical and numerical modeling techniques are typi-
cally used to determine well spacings, rates, and cleanup
times. A variety of techniques are available for this pur-
pose (U.S. EPA, 1985a). Analytical models typically in-
volve the application of the Theis equation or Darcy's law
or a variation thereof in simulating the response of an
aquifer to pumping. They are often referred to as capture
zone or wellfield simulation models. These models are
typically two-dimensional and usually involve many sim-
plifying assumptions. Therefore, it is often difficult to
simulate three-dimensional aspects of hydrogeologic set-
tings. This is particularly troublesome in complex settings
involving multiple layers, or where partially penetrating
wells and cutoff walls are to be designed. Analytical mod-
els are best suited for use in more homogeneous set-
tings. There are several commercially available analytical
models. The more accurate models incorporate image-
well theory, the principle of superposition, well hydraulics
equations, and particle-tracking analyses.
Depending on the site-specific design requirements, ana-
lytical models can be used to provide sufficient informa-
tion for screening and, sometimes, for design. They can
be used to provide rough estimates of well spacings and
pumping rates. This is often suitable for quick or interim
cleanup measures or for long-term measures where flexi-
bility to modify the design once the system has been in
operation has been incorporated into the design. How-
ever, care should be taken in applying these models to
complex hydrogeologic settings. The simulative capability
of the model may not be adequate for incorporating all of
the flow complexities and may result in gross errors in
design.
Numerical models typically offer more capability to simu-
late varying aspects of a hydrogeologic setting. These
models have the capability to model features such as ex-
traction wells, injection wells, impermeable boundaries,
recharge from surface water or precipitation, and multiple
aquifer systems. Both two-dimensional and three-dimen-
sional numerical models are available. Three-dimen-
sional models usually provide more representative results
due to the capability of simulating vertical variations in
flow properties. However, they are more complex and ex-
pensive to use. The predominant advantage of numerical
models over other analysis techniques is the ability to in-
corporate vast amounts of data and complex hydro-
geologic features into a framework that will provide
realistic appraisals of how effective a pump-and-treat
system may be. However, they are typically more expen-
sive and time consuming and require detailed site char-
acterization data to apply properly. Often, it is not feasible
to apply numerical models to the design of short-term
stabilization measures.
There are several questions to be answered when deter-
mining the required level of model. Figure 4-1 is a flow
chart that illustrates the hierarchy of decisions to be
made. Each question or decision must be answered in
the affirmative for analytical (Level I) modeling to be cho-
sen. A "no" answer at any decision point pushes the user
towards the use of a numerical (Level II) model, where-
upon, data and resource availability should be examined.
This hierarchy was developed to define the strict and lim-
ited conditions of analytical model use in remediation ac-
tion assessment. The user is referred to U.S. EPA
(1985a), for more information regarding model tech-
niques and selection.
If sufficient information is available, numerical models
should be utilized, particularly in the design of systems
under complex hydrogeologic and contaminant condi-
tions. The use of numerical modeling in designing pump-
and-treat systems usually involves four steps:
1. Development
2. Calibration
3. Sensitivity analyses
4. Design and operational simulations
Model development involves the input of aquifer proper-
ties and site features. The following information is usually
required as input:
59
-------
REASSESS
DATA NEEDS
i
_ ARE ORDER OF MAGNITUDE
""**' PREDICTIONS ACCL-P 1 ABLbV
rl
'I
IS IT REASONABLE TO ASSUME THAT MEDIA PROPERTIES
ARE UNIFORM, AND DO NOT VARY SPATIALLY?
NO I
f YES
IS IT REASONABLE TO ASSUME THAT THE
FLOW FIELD IS UNIFORM, STEADY, AND REGULAR?
NO I
f YES
IS IT REASONABLE TO ASSUME THAT THE
SITE GEOMETRY IS,REGULAR?
NO 1
f YES
1 ARE THE SELECTED REMEDIAL ACTIONS
| RELATIVELY SIMPLE IN CONFIGURATION?
NO 1
' f YES
DOES THE POLLUTANT HAVE RELATIVELY
THE SAME DENSITY AS WATER?
: Nฐ I
! t YES
DO YOU HAVE
SUFFICIENT RESOURCES
AND AVAILABLE DATA
FOR NUMERICAL MODELS?
USE LEVEL 1: ANALYTICAL MODEL
kh. IIQFIFVFI 11-NiiMFRl^Al MnnF
L
REFERENCE: US EPA, 1985a
Figure 4-1. Flow Chart to Determine the Level of Modeling Required.
60
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Delineation of model boundaries
Transmissivity
Storativity
Vertical hydraulic conductivity
B Aquifer thickness
H Surface water bodies
Pumping or.injection sources
Precipitation recharge
Potentiometric surface
Porosity
Dispersivity
Retardation factors or adsorption coefficients
Decay constants
Source strength and location
Initial concentrations
There are two basic formulations in numerical modeling:
finite difference and finite element. In both cases, a grid
system is developed. This involves discretizing the model
area into individual cells. The cells are typically rectangu-
lar, triangular, or quadrilateral. Once the grid system is
developed, the data are organized into arrays identifying
a parameter value at each cell.
The accuracy and the credibility of model predictions are
determined by the adequacy of calibration. This is par-
ticularly important in designing pump-and-treat systems,
as the accuracy of predicted capture zones are depend-
ent on how close the simulated response of the aquifer to
pumping is to actual field behavior. This process may in-
volve comparing field measured water levels, draw-
downs, and concentrations to model predictions. Often,
input parameters must be varied to provide a suitable
match. Visual and statistical comparisons are typically
used to describe the degree of calibration of the model.
Once a model is calibrated, it can be used to determine
well spacings, pumping rates, cleanup times, and impacts
of operational modifications. The actual use of the model
is often a trial and error exercise in which different con-
figurations of pumping and/or reinjection are simulated. If
a flow model is used, the different configurations can be
compared by comparing the resulting zones of capture. If
a contaminant transport model is used, the effectiveness
of the system can be evaluated through the prediction of
cleanup times or the effectiveness of capture. The model
can be used to optimize the system from a technical and
cost standpoint. Once the system is operational, field
monitoring data can be used to refine the model's predic-
tive capability through additional calibration. The model
can then be used to simulate operational changes and
impacts of operational changes on cleanup.
4.2 GROUND-WATER TREATMENT OPTIONS
Numerous treatment technologies exist for the remedia-
tion of contaminated ground water. Generally, most con-
ventional treatment technologies traditionally used for
industrial wastewater treatment are applicable to treat-
ment of contaminated ground water. The selection of the
most appropriate technology or series of technologies re-
quires careful consideration of the type and concentration
of contaminant(s) in the ground water, and the discharge
requirements for the treated water. Ambient water quality
characteristics important to process design, such as iron
and manganese occurrence, should not be overlooked.
Often a treatment train consisting of several technologies
in a series is needed.
A number of ground-water treatment technologies and
their applications are presented in Figure 4-2. The intent
of this figure is to provide a representative sampling of
technologies that may be used for ground-water treat-
ment during a stabilization action. It is not meant to imply
that other technologies could not be used. The treatment
technologies presented in Figure 4-2 can be applied
whether treating surface water or ground water.
Due to the proven status of these treatment technologies
and since an extensive amount of literature is available
that describes the technologies and their uses, the reader
is referred to several U.S. EPA guidance documents,
among others. Recommended reference documents in-
clude the Guidance on Remedial Actions for Contami-
nated Ground Water at Superfund Sites (U.S. EPA,
1988a), and Technology Transfer HandbookRemedial
Action at Waste Disposal Sites (U.S. EPA, 1985a). Pat-
terson (1985), is an excellent textbook for screening tech-
nologies since it is organized by contaminant rather than
by technology. Information useful for designing ground-
water treatment systems can be found in Eckenfielder
(1989). Other recommended references are identified be-
low.
4.3 REFERENCES
Eckenfielder, W.W. 1989. Industrial Water Pollution Con-
trol. 2nd edition. McGraw-Hill, New York.
Goveland, S.M. 1982. Optimal Dynamic Management of
Groundwater Pollutant Sources. Water Resources Re-
search. Vol. 18, No. 1, February.
Lyman, W.J., W.F. Reehl, and D.H. Rosenblatt. 1982,
Handbook of Chemical Properties Estimation Methods.
McGraw-Hill, New York, NY.
Ohneck, R.J. and G.L Gardner. 1982. Restoration of an
Aquifer Contaminated by an Accidental Spill or Organic
Chemicals. Presented to 2nd National Symposium on
Aquifer Restoration and Ground-Water Monitoring,
May 26-28.
61
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CONTAMINANTS
Metals
1 leavy Melals
1 lex Chromhim
Arsenic
Mercury
Cyanide
Corrosives
Volatile Organics
Kclonos
Semivolatile Organics
Pesticides
PCBs
Dioxins
Oil a Grease/Moating Products
Applicable
X
X
X
X
X
X
X
X
X
X
X
X
o
X
X
X
o
o
X
X
X
X
X
o
o
X
X
0
X
X
o
o
X
X
X
o
X i
x :
!
i
X i
i
o
ฎ
X
e
X
X
X
X
X
X
X
X
X
X
X
X
X
X
o
X
X
X
X
X
X
o
X
X
X
X
X
X
X
X
X
X
X
o
X
X
X
0
o
o
X
X
X
X
X
X
X
X
X
0
o
X
0
o
X
X
X
X
o
o
X
X
X
X
X
X
X
X
o
o
o
o
X
o
X
X
o
X
ซ
X
X
X
X
X
X
X
X
X
X
o
X
o
o
X
X
0
X
X
X
X
X
X
O Potentially Applicable '
X Not Applicable ;
References: U.S. EPA, 1986; U.S. EPA. 1989; U.S. EPA; 1985, Patterson, 1985; U.S. EPA, 1990
Note: "Technology includes several processes; reverse bsmosis and ultrafiltration among others.
Figure 4-2. Applicability of Treatment Technologies to Contaminated Ground Water.
Patterson, J.W. 1985. Industrial Wastewater Treatment
Technology, 2nd edition. Butterworths Publisher. |
Roberts, P.V., P.L. McCarty, and W.M. Roman. 1978.
Direct Injection of Reclaimed Water Into an Aquifer.
Journal of the Environmental Engineering Division, Proc.
ASCE, October. i
U.S. EPA. 1991. Remedial Action, Treatment, and Dis-
posal of Hazardous Waste. EPA/600/9-91/002. Office of
Research and Development. ',
U.S. EPA. 1990a. Basics of Pump-and-Treat Ground
Water Remediation Technology. EPA/600/8-90/003. :
U.S. EPA. 1990b. The Superfund Innovative Technology
Evaluation Program: Technology Profiles. EPA/540/5-
90/006. Office of Solid Waste and Emergency Response.
U.S. EPA, 1989. Performance Evaluations of Pump-and-
Treat Remediations. EPA/540/4-89/005. Office of1 Re-
search and Development. ;
U.S. EPA. 1988a. Guidance on Remedial Actions for [Con-
taminated Ground Water at Superfund Sites. EPA/540/G-
88/003. Office of Emergency and Remedial Response.l
U.S. EPA. 1988b. Technology Screening Guide forTreatr
ment of CERCLA Soils and Sludges. EPA 540-2-88-004.
U.S. EPA. 1986a. Mobile Treatment Technologies for Su-
perfund Wastes. EPA 540/2-86/003(f). Office of Solid
Waste and Emergency Response.
U.S. EPA. 1986b. Treatment Technology BriefsAlterna-
tives to Hazardous Waste Landfills. EPA/600/8-86/017.
Hazardous Waste Engineering Research Laboratory,
ซ>
U.S. EPA. 1985a. Technology Transfer HandbookRe-
medial Action at Waste Disposal Sites.U.S. EPA/625/6-
85/006. Office of Emergency and Remedial Response.
U.S. EPA. 1985b. Modeling Remedial Actions at Uncon-
trolled Hazardous Waste Sites. EPA/540-2-85/001.
U.S. EPA. 1983. Methods for Chemical Analysis of Water
and Wastes. EPA-60/4-79-020.
U.S. EPA. 1982. Evaluation of Landfill Remedial Action
Alternatives through Groundwater Modeling. EPA-600/9-
82-002.
Weston, Inc. 1989. Remedial Program EvaluationGil-
son Road Site. Nashua, New Hampshire. Prepared for
NHDES. February.
62
U.S.G. P.O.: 1991 -548-187 -.40613
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