En*'1
no
--'y '986
DC 20«60
Criteria for Identifying
Areas of Vulnerable
Hydrogeology Under the
Resource Conservation
and Recovery Act
Statutory Interpretive
Guidance
Guidance Manual
for Hazardous Waste
Land Treatment, Storage
and Disposal Facilities
Interim Final
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GUIDANCE CRITERIA FOR
IDENTIFYING AREAS OF VULNERABLE HYDROGEOLOGY
UNDER THE RESOURCE CONSERVATION AND RECOVERY ACT
RCRA STATUTORY INTERPRETIVE GUIDANCE
Guidance Manual for Hazardous Waste Land
Treatment, Storage, and Disposal Facilities
Interim Final
Office of Solid Waste
Waste Management Division
U.S. Environmental Protection Agency
401 M Street, S.W.
Washington, D.C. 20460
July 1986
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NOTICE
This document has been reviewed by the Science Advisory
Board to the Environmental Protection Agency at the request of
Marcia E. Williams, Director, Office of Solid Waste. The Board
is a public advisory group comprised of expert scientists that
provides extramural scientific information and advice to the
Administrator and other officials of the Environmental Protection
Agency. The Board is structured to provide a balanced expert
assessment of scientific matters related to problems facing the
Agency.
The Environmental Engineering Committee of the Board
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reviewed this document in draft form between December 1985 and -'
April 1986 and provided comments in a report titled "Report on
the Review of the Permit Writers' Guidance Manual for the Location
of Hazardous Waste Land Treatment, Storage and Disposal Facilities,
Phase II." Comments presented in that report were considered and
revisions to the draft were made in preparing this document.
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ACKNOWLEDGEMENT S
This man'ual was prepared by the Waste Management Division of
the Office of Solid Waste. Glen R. Galen was the project officer
and principal 'editor. Arthur Day, program manager, and George
Dixon and Lauris Davies of the Land Disposal Branch also made
major contributions to this manual.
Consultants played significant roles in the preparation of
the guidance manual. William Doucette and Donald Lundy of
Geraghty & Miller, Inc., and Charles Young, Diane Heineman,
Alfred Leonard, Pablo Huidobro, Steven Konieczny, Michael Mills,
John Rand, and Robert Clemens of GCA Corporation Technology
Division, developed technical sections of the manual and provided
technical assistance and background information. Joe English
and Chris Eddy of Battelle Northwest developed the Technical
Resource Documents attached as Appendices to this guidance manual.
Teff Goodman and his staff at ICF, Incorporated conducted an
impact analysis of the ground-water vulnerability criteria.
Keros Cartwright of the Illinois Geological Survey served as
technical advisor to the project and provided thoughtful comment
on the approach to location evaluation. Elizabeth Marcotte
and Eric Hillenbrand of Sobotka and Company provided assistance
in preparing the final version of the manual. *
Special thanks are expressed to the personnel of EPA Regional
Offices in Regions I, III, IV, V, VI, VII, and IX for technical
assistance and support in preparation of the background informa-
tion for Case Study Appendix E, and for peer review and work
group participation in developing the manuals. Special thanks
are also expressed to Harry Torno and members of the Environmental
Engineering Committee of the EPA Science Advisory Board for their
thoughtful review and commentary.
Special thanks are also expressed to Valerie Holloway,
Kathryn Schmitz, and Audrey Smith of the EPA Office of Solid
Waste for their assistance in the preparation of this document.
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EXECUTIVE SUMMARY
Section 3004(o)(7) of the Resource Conservation and Recovery
Act (RCRA), as "amended by the Hazardous and Solid Waste Amendments of
1984 (HSWA), requires the Environmental Protection Agency (EPA) to:
o Publish guidance criteria identifying areas of
vulnerable hydrogeology.
o Promulgate regulations specifying criteria for the
acceptable location of new and existing hazardous
waste treatment, storage, and disposal (HWTSD)
facilities as necessary to protect human health and
the environment.
This guidance document responds to the first requirement -- the
development of criteria for identifying areas of vulnerable
hydrogeology. EPA is developing options to respond to the second
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requirement and expects to propose regulations in September 1987.
Section 3 of this guidance document specifies the method
developed by EPA for determining ground-water vulnerability at
hazardous waste facilities.£/ This method requires the calcula-
tion of the time of travel (TOT) of ground water along a 100-foot
flow line originating at the base of a hazardous waste unit;
Figure ES-1 shows an example of such a flow line. The 100-foot
flow line distance provides a representative "sample" of the
geologic materials at the site and represents a distance that
I/ The criteria are applicable only to hazardous waste
facilities regulated under RCRA, and are not intended for use
under any other statutory program, such as land application
of pesticides. This document does not apply to "land treatment
units" (or "land farms") regulated under 40 CFR 264 or 265 Subpart
M, which characteristically utilize site soils as a treatment
medium in which degradation, transformation, or immobilization of
hazardous constituents occurs. Rather, this document applies to
land-based treatment, storage, and disposal units (surface impound-
ments, waste piles, and landfills) regulated under 40 CFR 264 or
265 Subparts K, L, and N.
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FIGURE ES-1: DIAGRAMMATIC CROSS SECTION OF MULTIPLE GEOLOGIC
STRATA SHOWING SELECTED FLOW LINE BENEATH A
HAZARDOUS WASTE UNIT
HAZARDOUS WASTE
UNIT
SELECTED FLOW LINE
GROUND WATER TABLE
V ' ' .' ' ' ' ' '-J 'ill' r
.., . ., ... ,... .
, ; -a ".-. a....-.. V ..-.-.....,. =-.
- -.»....:..-.-?.-:-.
' '."' -' -a'-.'.*'." ! '' V '-' -°'.-'' » '*;'.''''? '-'«. ' -"'.o-''.o .- *
A, B, AND C ARE SEGMENTS OF THE SELECTED FLOW LINE
A + B + C = 100 FEET
EACH GEOLOGIC UNIT HAS DIFFERENT PROPERTIES THAT DETERMINE
FLOW LINE DIRECTION
ES-2
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a permit applicant is likely to have investigated in collecting
hydrogeologic .data necessary to meet the permit application
information requirements contained in 40 CFR 270.14(c) for preparing
a Part B permit application.
Locations with short TOTs (i.e., where the ground water
moves fairly rapidly) are considered more vulnerable than those
with long TOTs. In general, EPA uses a TOT along a 100-foot flow
line (abbreviated "TOTioo") on the order of 100 years as the
criterion for determining vulnerability. Sites having units
used for the disposal of hazardous waste are vulnerable if the
ground water takes less than on the order of 100 years to travel
100 feet; sites at which wastes are certain to be removed from
land-based treatment or storage units at closure are vulnerable
if their TOTioo ^s less than the time needed to implement a
corrective action.
An evaluation of many hydrogeologic settings indicates that
TOTiQO values tend to cluster at various points within the
continuum of possible values. This clustering of settings indicates
that degrees of vulnerability exist, with locations characterized
by very low TOTioo values being more vulnerable than those with
higher values.
EPA expects that a small number of locations that marginally
pass or fail the tests will exist. By using values of a general
order of years rather than a specific number of years for the teat,
EPA is providing the permit writer with discretion both in inter-
preting the test results and in making a final determination on
vulnerability. Situations where units marginally pass or fail
ES-3
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the test will require that the permit writer evaluate site-specific
conditions, reliability of the sampling data and means of collection
used to support the finding, and the basis for the final TOT value
reported in more detail.
In constructing flow lines and calculating TOT values using
this method, migration through the unsaturated zone is generally
not considered in the calculation because at the locations of
most existing facilities (in humid areas where recharge to ground
water results in a high water table) the effect of the unsaturated
zone on TOT is negligible. However, in areas with arid or semi-
arid climates and thick unsaturated zones, the unsaturated zone
may significantly affect TOT and should be considered in the TOT
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calculation. A method is provided in Appendix C to this document
for incorporating unsaturated zone flow in TOT calculations.
The TOT concept integrates several hydrogeologic parameters
into a single measure chat reflects the potential for pollutant
migration and subsequent human and environmental exposure. EPA
developed the method for use with well-prepared, complete permit
application data. Its usefulness and reliability depend heavily
on the accuracy of application data for hydraulic conductivity,
hydraulic gradient, and effective porosity (or gravity drainable
porosity) for the entire area of the site.
The Introduction (Section 1) to this document describes
how this guidance may relate to the hazardous waste permitting
program and other RCRA programs designed to protect ground water.
EPA recognizes that vulnerable hydrogeology cannot be the sole
determining factor in making permitting decisions regarding certain
ES-4
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Locations for the purpose of siting or in permitting existing facil-
ities. In general, the TOT method provides a trigger for more detail-
ed review and evaluation of sites that are identified as having
possibly vulnerable hydrogeologies. The extent ot site review
and evaluation necessary is related directly to the margin by
which the location fails or passes the vulnerability criteria.
Sites that fail by a wide margin would obviously require closer
examination than sites that are deemed non-vulnerable. The
results of this more detailed review may provide a basis for
eventual permit conditions or modifications in design or operating
practices. In addition, the Agency is examining hydrogeologic
vulnerability and its relationship to corrective action requirements,
»
specific facility design and operating standards, and the hazardous
waste land disposal restrictions provision of HSWA in making
permitting decisions; the conclusions of this study will serve as
the basis for future rule and guidance development.
Section 2 explains EPA's reasons for selecting the TOT
method as the basis for determining vulnerability. The TOT
calculations can be used to identify those locations that minimize
the potential size of a contaminant plume and maximize the time
before a release from a unit can reach an aquifer. In addition,
the TOT test results, as opposed to criteria based upon a risk
analysis or environmental performance approach, can be calculated
with relative ease using complete Part B permit application data.
Section 3 presents technical procedures for performing the
TOT calculation. Section 4 describes situations where special
engineering modifications, such as installing grout curtains,
ES-5
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might be used to enhance TOT. Section 5 provides abstracts of
supporting documents and references.
i.
Several important technical references are included as
appendices to this document. Appendix A, entitled Methods for
Evaluating Hydrogeologic Parameters, describes how to determine
hydraulic conductivity, gradient, and effective porosity. Appendix
B describes how to construct flow nets to understand ground-water
flow patterns at hazardous waste facilities. Appendix C describes
how TOT can be estimated for the unsaturated zone in areas with
arid or semi-arid climates where thick unsaturated zones are common.
Appendix D describes technical analyses of the TOT tests perform-
ed by OSW. EPA has assessed the utility of the TOT tests to
identify hydrogeologic settings that minimize the potential for *
exposure to releases of hazardous waste via ground water with risk
analyses using actual site performance data and theoretical modeling.
Preliminary analysis of 228 HWLTSD facilities indicates that
approximately 72 percent of this population is located in a
vulnerable hydrogeology. T-he Office of Solid Waste is continuing
its verification analysis of the TOT tests by studying facility
performance in a range of geologic settings.
Constructing flow nets for a site and using the vulnerable
hydrogeology calculation can provide a tool for ensuring that a
location is properly characterized and the ground water at the
site can be properly monitored. Site Characterization and the
Ability to Monitor are two criteria discussed in the Phase I
Location Guidance Manual that the facility owner or operator must
meet as required by RCRA.
The TOT method is also useful in identifying the potential
ES-6
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for the "basement seepage pathway," which is one way by which the
public can be exposed to contaminated ground water. Once this
pathway is identified, it might be minimized or eliminated
by making certain facility design modifications. The basement
seepage pathway is discussed in further detail in Section 2,
as well as in Section 3.7 of Appendix D.
Alternate Concentration Limits (ACLs) are granted by EPA
through the permitting process under RCRA Parts 264 and 270 and
are established in the context of the facility Ground-Water
Protection Standard under Section 264.92. The finding that an
ACL is warranted at a specific facility is based on a sophisticated,
site-specific analysis. Because of this, the Agency considers an
approved ACL demonstration as taking precedence over a determina-
»
tion that the facility is in a vulnerable hydrogeology.
This document is being released as "Interim Final" Guidance
to allow EPA to revise the manual if technical inaccuracies exist.
An earlier draft (November 1985), referred to as the "Phase II"
Location Guidance Manual, was reviewed by the Environmental
Engineering Committee of the EPA's Science Advisory Board. This
Vulnerable Hydrogeology Guidance reflects the Committee's comments
and replaces the draft Phase II.
EPA will separately publish a notice in the Federal Register
at a future date announcing the availability of a RCRA Technical
Resource Document entitled, Methods for Evaluating Facility
Location and Facility Case Studies. This document is a technical
illustration of the application of both the TOT method and four
ES-7
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criteria for facility location published by EPA in February 1935
in the document entitled, Permit Writers' Guidance Manual for
^
Hazardous Waste Land Storage and Disposal Facilities - Phase I
Criteria for Location Acceptability and Existing Applicable
Regulations.
ES-8
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CONTENTS
Page
Executive Summary ES-1
Tables ii i
Figures 11 i
1. 0 Introduction 1-1
1.1 The Integrated OSW Ground-Water Strategy 1-3
1.2 Intended Use of. This Document 1-6
2.0 Technical Background 2-1
2.1 Necessary Characteristics of Methods for
Identifying Vulnerable Hydrogeology 2-2
2.2 Approaches Considered by EPA 2-3
2.2.1 Parametric Criteria 2-4»
2.2.2 Risk or Environmental Performance
Criteria 2-5
2.2.3 Integrated Criteria.. 2-6
2.3 Time of Travel (TOT) Tests for Identifying
Vulnerable Hydrogeology 2-8
2.3.1 Distinction between Treatment or Storage
Units and Disposal Units 2-12
2.3.2 Use of Engineered Barriers 2-13
3.0 Technical Methods to Determine Ground-Water
Vulnerability 3-1
3.1 Influence of the Unsaturated Zone 3-2
3.2 Time of Travel (TOT) Analysis 3-2
3.2.1 Hydraulic Conductivity 3-4
3.2.2 Hydraulic Gradient 3-9
3.2.3 Effective Porosity 3 .1
3.2.4 Necessary Precautions 3-12
3.3 Modifying TOTioo for Data Reliability 3-14
3.4 Use of Additional Containment 3-21
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Page
4.0 Applications of Engineered Structures 4-1
4.1 Slurry, Walls 4-2
4.2 Grout Curtains 4-3
4.3 Conditions Appropriate For Applying
Engineered Structures to Adjust TOT 4-4
5.0 Supporting Documents and References 5-1
5.0 Supporting Documents 5-1
5.2 References 5-5
Appendices
A. Technical Methods for Evaluating
Hydrogeologic Parameters »
B. Ground-Water Flow Net/Flow Line Construction
and Analysis
C. Technical Methods for Calculating Time of Travel
(TOT) in the Unsaturated Zone
D. Development of Vulnerability Criteria Based on
Risk Assessments and Theoretical Modeling
11
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TABLES
Number Page
3.2-1 Hydraulic Conductivity Conversion Factors ..... 3-8
3.2-2 Default Values for Effective Porosity for
Use in Time of: Travel Analysis ................ 3-13
3.3-1 Reliability Factors for Adjusting the
Calculation of TOTiQO ......................... 3-15
3.3-2 Reliability of Tests for Determining
Hydraulic Factors ............................. 3-17
FIGURES
ES-1 Cross Section of Multiple Geologic Strata
Showing Selected Flow Line Beneath Facility... ES-2
3.2.1-1 Cross Section of Multiple Geologic Strata
jhowing Selected Flow Line Beneath Facility... 3-7
3.2.2-la Selecting Initial Values for Head and the
Flow Line (Surtace Impoundment) 3-10
3.2.2-lb Selecting Initial Values for Head and the
Flow Line (Landfill) 3-10
3.2.2-lc(l) Selecting Initial Values for Head and the
Flow Line Without Liner (Waste Pile) 3-10
3.2.2-lc(2) Selection Initial Values for Head and the
Flow Line with Liner (Waste Pile) 3-10
111
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1.0 INTRODUCTION
Section 3004(o)(7) of the Resource Conservation and Recovery
Act (RCRA), as amended by the Hazardous and Solid Waste Amend-
ments of 1984 (HSWA), requires EPA to:
o Publish guidance criteria identifying areas of
vulnerable hydrogeology.
o Promulgate regulations specifying criteria for the accept-
able location of new and existing hazardous waste
treatment, storage, and disposal (HWTSD) facilities as
necessary to protect human health and the environment.
EPA has prepared this manual in response to the first of these
requirements. EPA is studying options to respond to the second
of these requirements and intends to propose regulations specifying
criteria for acceptable locations of new and existing HWTSD »
facilities by September 1987.
The purpose of this manual is to provide RCRA permit writers
with a standardized technical method (or technical "guidance
criteria") for evaluating hydrogeologic data submitted in permit
applications for hazardous waste land treatment, storage, and
disposal facilities^/ to determine if the facilities are located
in "areas of vulnerable hydrogeology." Although not specifically
defined in HSWA, EPA considers "areas of vulnerable hydrogeology"
I/ The criteria are applicable only to hazardous waste
facilities regulated under RCRA, and are not intended for use
under any other statutory program, such as the land application
of pesticides. This document does not apply to "land treatment
units" (or "land farms") regulated under 40 CFR 264 or 265 Subpart
M, which characteristically utilize site soils as a treatment
medium in which degradation, transformation, or immobilization of
hazardous constituents occurs. Rather, this document applies to
land-based treatment, storage, and disposal units (surface
impoundments, waste piles, and landfills) regulated under 40 CFR
264 or 265 Subparts K,L, and N.
1-1
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to be areas in which the predominant natural hydrogeologic conditions
are conducive to the subsurface migration of contaminants in a
manner that may adversely affect drinking-water sources, sensitive
ecological systems, or nearby residents. EPA believes that the
intrinsic hydrogeologic vulnerability of a HWLTSD facility's
location is one of several key determinants of the facility's
long-term success in minimizing contaminant migration (other
determinants include, for example, leaching properties of the
waste). If both the engineered components (e.g., liners and cap)
and the ground-water monitoring and response program should fail
in some way at a facility, the effects of the resulting release
of hazardous constituents on human health and the environment
t
might differ significantly depending upon the hydrogeologic
vulnerability of the location.
EPA intends to incorporate consideration of the hydrogeolo-
gic vulnerability of a facility's location into RCRA permitting
decisions. Currently, EPA will be able to do 'so to only a limited
extent 'due to the constraints of existing regulations. However,
once regulations specifying criteria for acceptable location
required under HSWA §30Q4(o)(7) have been promulgated, £PA will
have much greater flexibility in considering the hydrogeologic
vulnerability of a facility's location in permitting decisions.
EPA is currently developing a strategy for integrating
existing and planned regulations and guidance developed or being
developed in response to HSWA provisions (including minimum
technological requirements, land disposal restrictions, location,
and corrective action) by determining their relationship to one
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another ana the Agency's Ground-Water Protection Strategy. when
completed, this. Integrated OSW Ground-Water Strategy will outline
EPA's policy under existing regulations for evaluating permitting
situations where either considering variances to standards or
applying additional permit conditions may be appropriate, based
upon the characteristics of the facility, its location, and the
wastes accepted. The Strategy will also provide policies to be
considered during future rulemaking, including that for Location
Standards.
The Integrated OSW Ground-Water Strategy, and certain ways
in which hydrogeologic vulnerability may eventually be considered
in permitting decisions, are discussed briefly in Section 1.1
below. Section 1.2 discusses how the permit writer should use *
this document pending completion of the integrated strategy and
promulgation of regulations for the location of HWLTSD facilities.
1.1 THE INTEGRATED OSW GROUND-WATER STRATEGY
OSW is developing an Integrated Ground-Water Strategy that
will, among other things, consider how hydrogeologic vulnerability
may be applied to facility permitting decisions. In addition, it
will evaluate the relationship of each of the major components of
the RCRA program mandated by HSWA. The Integrated OSW Ground-Wate;
Strategy is, in part, a response to the Agency's 1984 Ground-Water
Protection Strategy (GWPS), which called for program offices to
develop policies for ground-water protection against a broad
framework of ground-water classification and protection. Although
OSW continues to support the ground-water classification framework
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proposed by the GWPS, the passage of HSWA necessitates a RCRA
$
Strategy implementing that framework that differs from the examples
provided in the 1984 Strategy (which predates HSWA).
Also in the context of the Integrated OSW Ground-Water
Strategy, OSW is considering whether the vulnerability criterion
might be used as a factor in considering waivers or variances
from existing liner design requirements. For example, impoundments
located in non-vulnerable areas might be candidates for a variance
from the interim status surface impoundment liner retrofitting
requirement as provided under §3005(j)(4). Likely, alternate
liner designs, compared with those specified in the manual entitled,
Minimum Technology Guidance on Double Liner Systems for Landfills^
t
and Surface Impoundments, EPA-OSW, May 24, 1985 (EPA/530-SW-85-01-4) ,
might be considered for non-vulnerable areas. Furthermore, OSW
is also considering using the vulnerable hydrogeology guidance
criteria as a factor in evaluations under the Hazardous waste
delisting petitions program, exposure assessment evaluations
under §3019, corrective actions under §3004(u), and closure plan
evaluat ion.
Regulations being developed to implement the land disposal
restriction and location criteria requirements of HSWA will
expand the regulatory basis for establishing additional permit
conditions as necessary to protect human health and the environ-
ment. The RCRA location standards will consider a number of
risk-related factors, as discussed in the Agency's Regulatory
Development Plan for Location Standards. Location factors under
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review include:
o the vulnerable hydrogeology criteria, reflecting the site's
natural ability to minimize release migration;
o the degree of current contamination;
o the feasibility of performing corrective action;
o ground-water use and value factors (e.g., the ground-water
classification framework of the EPA Ground-Water Protection
Strategy); and
o the relationship or representativeness of local versus
sub-regional hydrogeology.
OSW is considering whether the hydrogeologic vulnerability
criteria could be used to identify locations where additional
permitting standards would be adequate to provide additional
protection, or locations where waivers or variances are most
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appropriate. For example, the vulnerability criterion may be
used as a trigger for considering more stringent permit requirements,
such as:
o Precluding petition demonstrations under the
land disposal restrictions program in settings
characterized by complex hydrogeology.
o Specifying more stringent corrective action
requirements, and possibly waste removal, in
certain areas with highly vulnerable hydrogeology.
o Requiring waste removal at closure (clean closure)
in certain sensitive or highly vulnerable locations.
o Requiring more stringent unit designs and operating
controls in certain locations. For example, addi-
tional engineered barriers (e.g., grout curtains)
in areas where shallow subsurface flow of waste
constituents may present a risk via the "basement
seepage pathway" (This pathway is described briefly
in Section 2 and in Appendix D).
OSW has not yet completed its assessment of potential uses
for the hydrogeologic vulnerability criteria in evaluating
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additional permit conditions or requests for variances or waivers.
The above examples of possible additional variances or require-
ments have been provided for illustrative purposes, and may not
be included in final policy on the use of the criteria.
1.2 INTENDED USE OF THIS DOCUMENT
This document provides technical guidance criteria, in the
form of a uniform method, to be used by permit writers in evaluating
hydrogeologic data necessary to meet the permit application inform-
ation requirements contained in 40 CFR 270.14(c) and submitted in
RCRA Part B permit applications for hazardous waste facilities.
Permit writers should use this method as guidance for determining
the adequacy of the permit applicant's site characterization,
evaluating the applicant's plans to monitor ground water at the
site, determining the adequacy of hydrogeologic data submitted in
permit applications, and identifying RGRA facilities located in
areas of vulnerable hydrogeology.
The evaluation of flow lines at the location, together with
the use of the TOT criteria, provide an excellent means of verify-
ing whether the location can be properly characterized and whether
the hydrogeologic conditions at the site are appropriate for
proper ground-water monitoring. Permit writers are encouraged
to use the guidance provided in the document and its appendices
to construct ground-water flow nets for facility locations as
a means of performing these activities, and to request, where
appropriate, that applicants install additional piezometers as part
of any further hydrogeologic studies they plan to perform in order
1-6
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to verify hydrogeologic information provided in the application.2/
Where results of the method indicate that a facility is
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located in an area of potentially vulnerable hydrogeology, the
permit writer should undertake a more detailed evaluation of
site characteristics and of the mitigation techniques proposed
by the owner or operator, or require additional permit conditions
or a contingent corrective action plan using existing regulatory
authority under RCRA, to assure that ground-water contamination
will be prevented or responded to quickly. Thus, the results
obtained using this method should be used as a screen to deter-
mine the need for and degree of a detailed evaluation of site
hydrogeology and of additional means for preventing ground-water
>
contamination which may be necessary at the facility.
Additional permit conditions for facilities located in
areas of vulnerable hydrogeology would not be necessary in the
case where an owner or operator has successfully demonstrated
that alternate concentration limits (ACLs) in "the Ground-Water
Protection Standard are justified. ACLs are granted by EPA through
the permitting process under RCRA Parts 264 and 270 and are
established in the context of the facility Ground-Water Protection
Standard under Section 264.92. The finding that an ACL is
warranted at a specific facility is based on a sophisticated,
site-specific analysis. Because of this, the Agency considers
an approved ACL demonstration as taking precedence over a
determination that the facility is in a vulnerable hydrogeology.
^/Additional hydrogeologic studies should not be required
or requested by the permit writer solely for the purpose
of considering the question of vulnerability.
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2.0 TECHNICAL BACKGROUND
The common'definition of vulnerable is "capable of being
physically wounded" or "open to attack or damage." This meaning
may be applied to the concept of ground water and how vulnerable
it is to contamination in various locations having certain hydro-
geologic conditions. EPA considers ground water to be vulnerable
if it is subject to "damage" by the introduction of contaminants
that may easily enter the ground water and affect drinking-water
sources, sensitive ecological systems, and other exposure pathways.
EPA is concerned that ground-water contamination, and
potentially resulting human and environmental exposure, can occur
as a result of failures of engineered controls and barriers, frop
human oversight, or after the end of the post-closure care period
established by the current facility permitting standards. Once
such contamination occurs, it can be both difficult and costly to
clean up.
Determining how ground- water can be vulnerable to contamination
involves understanding the potential human and environmental path-
ways for exposure to such contamination. As the case studies in
Appendix D illustrate, there are three general forms of potential
exposure to contaminated ground water. The first is the well-
recognized water well pathway, in which an aquifer contaminated
by hazardous waste leachate is used to supply water for residential,
commercial, agricultural, or industrial uses. The second means
of exposure can occur when contaminated ground water discharges
to surface waters, thereby endangering both the surface-water
ecosystem and water users downstream. The third is herein termed
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the basement seepage pathway. Human exposure can occur when
* " contaminated ground water seeps into residences through utility
line apertures or walls. When these structures are permanently
or seasonally affected by a shallow saturated zone which can be
contaminated by releases from a waste management unit, the poten-
tial for contamination via the basement seepage pathway exists;
this pathway is illustrated in Section 3.7 of Appendix D.
EPA believes that the ground-water vulnerability definition
should respond to concerns for all three general pathways. EPA
has investigated several methods or tests that could be used to
identify vulnerable hydrogeology. The following sections discuss
the criteria EPA used in evaluating these methods, the methods
considered, and the method selected. t
2.1 NECESSARY CHARACTERISTICS OF METHODS FOR IDENTIFYING
VULNERABLE HYDROGEOLOGY
To evaluate alternative methods for identifying areas of
vulnerable hydrogeology, EPA first determined the characteristics
that an acceptable method should have in order to be useful within
the structure of the permitting program. EPA decided that the tests
should have the following characteristics:
o Their use should generally require no more information
about the facility than that already required in Part B
permit applications (see Section 270.14(c) for permit
application information standards related to ground water).
o They should not require predictive analyses that require
(1) unreliable estimates of technical parameters, and
(2) parameters that the permit applicant or permit writer
would find difficult to obtain.
o They should be reasonably predictive of results of more
data-intensive analyses that could be used to evaluate
hydrologic, geologic, and pedologic conditions at the
facility location. They should also be flexible enough
to make use of additional data that the permit applicant
may provide to supplement a permit application.
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o They should be consistent with EPA's overall policy
for facility siting, which is that the location should
serve t^o minimize the potential for exposure to wastes
and constituents released as a result of the failure of
engineered containment barriers such as liners and caps,
or as a result of failure to operate the facility in
accordance with standards.
o They should not unduly restrict the ability of the regu-
lated community to find locations that conform to the
definition. Nonvulnerable settings should be available
within each major region of the Nation. An overly
conservative definition that excluded all but a very small
percentage of the land area, particularly if this area was
found only within one Region, could encourage illegal
disposal. This would obviously be counterproductive to
EPA policy to minimize the potential for exposure to wastes
o They should not unduly conflict with existing State
siting criteria.
o Their use should be within the technical capabilities
of owners and operators, professional consultants, and
EPA Regional Office and State Agency permit writers. t
2.2 APPROACHES CONSIDERED BY EPA
EPA examined the suitability of three general approaches
against certain characteristics, which were:
o Parametric Criteria: Ground-water vulnerability
would be based on one or more hydrogeologic param-
eters, such as maximum acceptable soil permeability
or minimum acceptable depth to the water table
beneath the hazardous waste management unit.
o Risk or Environmental Performance Criteria: A
non-vulnerable location would be identified when a
site-specific technical demonstration showed that the
concentrations of hazardous constituents (or some
surrogate) measured at some point of concern would be
within certain environmental performance or health
risk limits in the event of a release of hazardous
constituents to the ground water.
o Integrated Criteria: Under this approach, a number of
closely related geologic and hydrogeologic parameters
would be collectively analyzed, with a result that
describes the ability of the location to minimize the
potential for exposure to releases.
Each of these approaches is discussed in more detail below.
2-3
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2.2.1 Parametric Criteria
EPA does not believe that the use of a single geologic or
hydrogeologic parameter is the best way to meet the overall objec-
tive of the vulnerability criterion, which is to minimize the poten-
tial for exposure. Using single properties of a location alone,
e.g., depth to ground water, to characterize a setting provides
limited technical information that would be inadequate upon which
to base a decision about vulnerability of ground water. Single
properties cannot be easily isolated from other hydrogeologic
factors needed to describe the performance characteristics of a
geologic terrain.
EPA examined criteria used by States for controlling the
location of hazardous waste land disposal facilities. This ana-.-
lysis is available in the report entitled, Review of State Siting
Criteria for the Location of Hazardous Waste Land Treatment,
Storage, and Disposal Facilities (EPA, OSW, 1984, Draft Final).
A significant conclusion from this examination is that a definition
based on parametric criteria alone would be extremely difficult to
apply in many States. This is due to the fact that a number of
States have different numerical standards for the same parameter
(e.g., permeability, depth to water, soil texture). An EPA defini-
tion based on such parameters might conflict with existing State
programs and create difficulties for a State seeking authorization
under Section 3006 of RCRA, if EPA tests for vulnerability were
codified in a facility location standard. The States will
undoubtedly have to amend their regulations to adopt Federal rules.
2-4
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However, in instituting any new requirements EPA will attempt to
minimize conflicts with existing State programs.
2.2.2 Risk or Environmental Performance Criteria
Of the possible criteria that can be used to define a
vulnerable hydrogeology, EPA believes that well-conceived risk
performance criteria, if properly conducted and supported with
adequate data and analysis, can provide a sound basis for permit
decisions. However, due to its complexity and data intensiveness,
risk analysis may not always be a feasible means for determining
vulnerable hydrogeology. Risk or environmental performance
criteria require a predictive analysis of the fate and transport
of hazardous constituents from the releasing unit. Although such
t
analyses tor certain chemicals may be possible (i.e., an alterna-te
concentration limit (ACL) demonstration), most typically require
detailed knowledge of the chemical composition of the waste or
leachate as it enters the underlying soil or the ground water,
and a sophisticated understanding of attenuation reactions and
mechanical dispersion.
EPA rejected this approach because it is inconsistent with
most of the initial objectives for the definition of criteria for
a vulnerable hydrogeology (see Section 2.1). These analyses
commonly employ numerical modeling to solve the many equations
describing ground-water flow and solute transport. In addition,
a risk-based approach presumes an adequate understanding of the
toxicologic properties of all potential constituents in isolation
or in mixtures. While EPA has such understandings of toxicology
2-5
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for many chemical compounds regulated under other EPA programs,
its data base if incomplete for most hazardous waste streams ana
complex leachates. Regardless of the state of knowledge of toxi-
cology, the application of fate and transport analysis requires
sophisticated expertise and data typically unavailable in a permit
application. In addition, a number of key parameters used in the
analysis, such as constituent attenuation and dispersion, and the
nature of the source term (i.e., waste or leachate composition
and volume) must be estimated using professional judgment.
Compounding this is the fact that some attenuation reactions,
such as cation exchange, are reversible.
2.2.3 Integrated Criteria
\
EPA believes that an integrated approach is the best way to'
meet the objectives of the vulnerable hydrogeology definition. The
Time of Travel (TOT) test outlined by EPA in this manual and describ-
ed in detail in Section 2.3, integrates information on geologic
characteristics and the direction and rate of ground/water flow
that should be provided in a complete Part B permit application, as
required by the permit application information requirements under
40 CFR 270.14(c). The test does not require predictive analyses
of the fate and transport of waste constituents. It also does not
create a potential for significant conflict with State policies
because it establishes a method of site analysis that can be
easily applied to most (if not all) current State rules. Use of
the TOT method supports two of the location criteria developed by
EPA under existing regulations (i.e., the ability to characterize
a site and the ability to monitor a site), which were published in
2-6
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the Permit Writers' Guidance Manual for Hazardous Waste Land
Storage and Disposal Facilities - Phase I (EPA, OSW, 1984, Final
Draft), because -the information needed to address those criteria
is also used in the analysis of ground-water vulnerability.
The technical expertise needed to apply the definition is well
within the capabilities of permit applicants and those permit
writers who are already responsible for assessing compliance
with the ground-water monitoring standards.
Finally, EPA believes that the test method described in the
next Section is justified by analyses of exposure potential and
health risk using actual facility performance data and theoretical
modeling. As discussed in Appendix D, EPA has calibrated the
»
vulnerable hydrogeology definition using such performance data and
theoretical modeling. This calibration shows that the integrated
criteria approach (i.e., Time of Travel) is not arbitrary; rather,
it provides an initial indication of those locations at which the
potential for adverse exposure is significant and it can be
consistent with the results of more sophisticated analyses in
many situations.
The use ot an integrated approach to define the potential
for contaminants to enter the ground water is being used by other
o
Agency offices. For example, the Office of Pesticide Programs
and the Office of Ground-Water Protection are studying "DRASTIC,"
a standardized system for evaluating ground-water pollution potential
using hydrogeologic settings. The system has two major portions:
(1) the designation of large, mappable geologic units termed
2-7
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hydrogeologic settings, and (2) the superposition of a relative
rating system t&at incorporates several hydrogeologic factors.
2.3 TIME OF TRAVEL (TOT) TESTS FOR IDENTIFYING
VULNERABLE HYDROGEOLOGY
Certain hydrogeologic settings are characterized by conditions
that make the ground-water resource at a site particularly vulnerable
to contamination in the event of a release from a facility. EPA has
selected an integrated Time of Travel method as a means of rationally
considering the principal hydrogeologic parameters that determine
ground-water vulnerability to contamination using one unified calcu-
lation. In the TOT method, hydrogeologic vulnerability is determined
by calculating the TOT of water along the first 100 feet of a ground-
water flow line originating at the hazardous waste management uni*t.
This calculation, abbreviated as the TOT^QQ, requires data on hydrau-
lic conductivity (also otten called permeability), the hydraulic
gradient, and the effective porosity of sediments (or gravity
drainable porosity of rock).
Using this calculation, ground water at a site is characterized
as nonvulnerable to contamination from land-based hazardous waste
management activities by its natural hydrogeologic conditions if
these ground-water tlow conditions are characteristic of aquitards
(or in some senses, aquicludes); ground water is vulnerable if tlow
conditions are characteristic of aquifers. EPA has analyzed the
characteristic TOT^gg values of aquitards and has found that these
values are clustered around 100 years or more. Therefore, EPA
intends to use TOT^QO values on the order of 100 years or greater
as characteristic of aquitards. These conditions define non-vulner-
able hydrogeologies for RCRA hazardous waste land disposal facilities
2-8
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only. For those land storage or treatment facilities where it is
f "
certain that wastes will be removed at closure, the vulnerability
of the ground wa£er is related to the time that would be necessary
to correct a problem in the event that design and operating controls
in place at the facility failed. The objective of the TOT^QO calcu-
lation should be to determine whether contamination could migrate
beyond this distance in less time than is needed to effectively
recognize and respond to a release through successful implementation
of a corrective action plan under Section 264.100. This determina-
tion must be site-specific. Permit writers should be sure to tirst
determine that wastes and contaminated soils will indeed be removed
from the site at closure before considering a vulnerability for
TOT]_QO that is less than on the order of 100 years. Permit writers
should closely examine the owner or operator's closure plan and
financial assurance for closure in making this determination.
The general definitions of aquitard and aquifer are somewhat
imprecise because, when applied to a specific location, strata are
considered aquifers or aquitards based upon the relative flow
conditions in the strata at that location and on the ability of the
geologic materials to bear ground water.
An aquifer is defined in EPA's hazardous waste regulations at
40 CFR 260.10 as a geologic formation, group of formations, or part
of a formation capable of yielding a significant amount of ground
water to wells or springs. An aquitard is not similarly defined,
but is considered in the professional liturature to have a
permeability that is not sufficient to allow the completion of
production wells within it (Freeze and Cherry, Groundwater, 1979,
2-9
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pg.47). The term aquiclude, as classically defined, is a saturated
geologic unit that is incapable of transmitting significant
quantities of water under ordinary hydraulic gradients (Ibid).
Very few formations fit this definition, in that most formations
contribute ground water to regional flow systems over very long
(or geologic) periods of time. EPA considers the term "aquitard"
to best characterize geologic formations with TOTioO that are on
the order of 100 years or more.
As long as contaminated ground water is entrained within
the aquitard, human exposure via drinking water exposure should
not occur. Similarly, exposure through the "basement seepage"
pathway is unlikely, provided that there are no sand
lenses or soil fractures within the aquitard to transmit flow.
Such features should be identified during the flow-line character-
ization process described in Section 3; case studies in Appendix
D illustrate this analysis procedure.
EPA believes that aquitards minimize the potential for exposure
to wastes released from units as a result of failures of engineered
barriers or human action by acting as passive control systems.
A location with a short TOT implies both a higher rate of potential
migration of contaminants, and a higher volume of ground-water flow.
This is because both TOT (or seepage velocity) and flow volume
(or specific discharge) are directly proportional to hydraulic
conductivity and hydraulic gradient. Similarly, a location
characterized by hydrogeologic conditions that encourage a long
TOT minimizes the potential for exposure to a release both in
terms of time and volume of potentially contaminated ground water.
2-10
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A 100-foot flow line for the interval over which the travel
time calculation is to be made represents a distance that a permit
applicant is likely to have investigated for the Part B permit
application. This distance should provide a fairly representative
"sample" of the geologic materials at the site. Investigations
over a shorter distance might fail to encounter important geologic
units (e.g., sand lenses) or structures (e.g., fracture zones,
solution cavities) that if present, could influence the rate and
direction of ground-water flow. Investigations over a larger
interval might require an.applicant to gain access to adjoining
properties for test drilling. If access were refused, neither the
applicant nor EPA could reliably apply the definition to the site.
If the permit writer encounters a case where such non-vulnerable
aquitards overlie, for example, a major high yielding aquifer, then
data should be examined, as feasible, to determine the degree to
which the 100-foot distance is representative of the local ground-
water flow system, and hence, of risk of exposure.
EPA is further analyzing the time period used to characterize a
vulnerable hydrogeology by evaluating numerous facility case studies,
but considers the 100 year timeframe to be supported by analyses
already complete'.~" Appendix D describes this analytical work.
Generally, the analyses show that locations tested have clearly
either passed or failed the TOT tests by a wide margin. The
pattern evolving from the analyses completed to date show very
few locations that fall very close to the 100-year level.
However, EPA expects that a small number of locations that
marginally pass or fail the tests will exist. By using values of
2-11
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a general order of years rather than a specific number of years
for the test, EPA is providing the permit writer with discretion
both in interpreting test results and in making a final determina-
tion on vulnerability. Situations where units marginally pass or
fail the test will require the permit writer to evaluate site-
specific conditions, reliability of the sampling data and means of
collection used to support the finding, and the basis for the final
TOT value reported, in more detail. In certain hydrogeologic
settings that marginally pass or fail the TOT test, use of
additional containment barriers, such as slurry walls and grout
curtains, may be appropriate means of modifying facility design and
operation to enhance meeting the TOT criterion. Section 4 describes
»
the conditions where the permit writer may want to consider additional
containment barriers.
2.3.1 Distinction between Treatment or Storage Units and
Disposal Units
EPA believes that a distinction in the TOT tests for treatment
or storage facilities and for disposal facilities is warranted in
certain situations. The distinction is justified by a comparison
of potential exposure and health risk at sites where wastes are
present for a finite period with sites at which wastes are present
indefinitely. This analysis is described in Section 2.0.2 of
Appendix D.
A shorter time frame for land-based treatment or storage
units may be justified for several reasons. Wastes and contaminated
soils are removed from treatment or storage units at the time of
their closure while wastes remain in disposal units after closure.
2-12
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The owner or operator of a treatment or storage unit is present
at the facility during this operating period to conduct ground-water
monitoring and response activities. Thus, the potential for
exposure to contaminated ground water as a result of waste
constituent discharges from treatment or storage units during
their operating period should be less than the potential that
exists after the owner or operator's period of responsibility for
monitoring and response ends. This means that a greater
range of hydrogeolog ic conditions at locations reflected by th >
shorter TOT test time frame can be tolerated for treatment or
storage units. The criterion for treatment and storage units is
proposed as a margin of safety to identify locations where a
>
release could rapidly develop into a large, extensive plume of -'
contamination before a corrective action could be effective.
Instituting an effective corrective action program at any location,
be it a storage or disposal facility, will take some time, especially
in situations where a contaminant plume is large. The permit writer
should first determine that the unit will, in fact, be closed as a
storage unit, and not as a disposal unit. The time needed to de-
tect a release that requires corrective action, to modify the permit
to implement this action, and to have this action succeed are site-
specific. Times for these factors should be less than the TOT cal-
culation for the 100-foot flow distance at the site.
2.3.2 Use of Engineered Barriers
A significant issue is the extent to which special engineering
methods might be used to modify ground-water flow patterns and
velocity at a site, thereby changing the TOT at the site to bring
2-13
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the test value within a more acceptable range. The starting point
for the flow lines used in the TOT]_QO analysis is at the base of
the treatment, storage, or disposal unit. This point is beneath
or beyond any synthetic liner and leachate collection system. The
very purpose of the vulnerability criterion is to describe locations
that minimize the potential for exposure in the result of failure
of engineered containment barriers (i.e., synthetic liners, caps).
Thus, it would be inconsistent with this purpose to allow a time
credit for those barriers. It is also difficult to know whether
or when synthetic barriers might fail (i.e., release wastes
beyond the initial design specifications of the barrier).
The flow line should, however, include well constructed clay
liners that may underlie synthetic liners and leachate collectioA
systems as part of a composite liner system. While clay liners
are engineered structures, they are composed of materials that
are similar or identical to existing, naturally occurring soils
at the site. It would be inconsistent not to consider clay
barriers at a site in the TOT calculation solely because they
were placed there by human action rather than by natural occurrences.
Before considering any clay barrier, the material must meet the
minimum technology requirements and construction quality assurance
for clay liners. Minimum technology requirements are discussed
in the manual entitled, Minimum Technology Guidance on Double
Liner Systems for Landfills and Surface Impoundments, EPA-OSW,
May 24, 1985, (EPA/530-SW-85-014). Quality assurance for the
construction of clay liners is discussed in the manual entitled,
Construction Quality Assurance for Hazardous Waste Land Disposal
2-14
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Facilities, EPA-ORD/OSW, October 1985, (EPA/530-SW-85-021).
Additionally, the Agency believes that engineering methods
beyond those already required in the facility permitting stan-
dards can also be used to increase travel time at specific types of
locations. Section 4.0 describes those situations in detail. in
general, slurry walls or grout curtains can be considered for
use as passive barriers to flow at sites that predominantly meet
the TQTioO tests (i.e., within 25 percent) based solely on the
geologic terrain beneath the site. Some locations composed
predominantly of clayey sediments may have minor, thin sand
lenses interspersed through the clays. A slurry wall or grout
curtain used to impede flow through such small lenses could be an
acceptable engineered modification.
Engineered structures would not be acceptable in all loca-
tions because these structures may serve as only temporary measures
in reducing TOT where the site is predominantly unsuitable.
Ground-water flow in geologic settings characterized by complex
fracturing of rock and sediments, and thick layers of sand and
gravel can be temporarily adjusted by slurry walls or grout
curtains. Use of these devices might cause the location to
temporarily meet the TOTiQO tests. However, engineered structures
used in such predominantly unsuitable locations are likely to
eventually fail, and might not be adequately constructed, just as
synthetic liners and covers. These geologic settings would not
predominantly meet the TOT^QQ tests if engineered structures were
absent, and are not appropriate for considering engineered
modif ications.
2-15
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3.0 TECHNICAL METHODS TO DETERMINE GROUND-WATER VULNERABILITY
To determine ground-water vulnerability, a time-of-travel
(TOT) calculation is performed to estimate how long a contaminant
moving at the velocity of ground water will take to migrate the
first 100 feet along a ground-water flow line originating at
the base of the hazardous waste management unit. The 100-foot
distance is a criterion that is easily comprehended and that in
most cases should be within the facility boundary. The chosen
flow path should exhibit the fastest migration route; this
should represent the "worst case" condition at the location.
Appendix B describes how flow nets can be used to identify these
flow paths. Care should be exercised to ensure that the flow
»
net includes the effects of regional geologic structures (e.g.,"
large-scale fracture patterns or formation discontinuities),
so that these effects are integrated into the 100-foot flow
path to adequately represent migration at the site.
TOT along the potential constituent release flow path(s)
provides both a relative measure of the protection offered by a
natural ground-water flow system and a means of comparing the
range of natural hydrogeologic systems. Appendix D of this
document, and the Technical Resource Document entitled, Technical
Methods for Evaluating Facility Location, present TOT analyses
for selected case studies.
The following sections explain the use of the TOT tests,
and describe how initial calculations can be modified to account
for data reliability and additional engineered containment.
3-1
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3.1 INFLUENCE OF THE UNSATURATED ZONE
The calculation of time of travel in this guidance manual
is restricted to flow in the saturated zone. A determination
of flow velocities in the unsaturated zone requires extensive
characterization of soil moisture content, capillary pressure,
head (tension), and hydraulic conductivity relationships.
Typically, unsaturated hydraulic conductivity and effective
porosity values are orders of magnitude less than those values
measured under saturated conditions. However, information on
such unsaturated zone properties is not explicitly or routinely
required in a Part B application. If the depth to water is
relatively shallow, the permit applicant or permit writer can
assume that the thickness of the unsaturated zone does not
significantly influence the TOTi_nn calculation. The Agency
thinks that the contribution of the unsaturated zone to time of
travel is insignificant at most facility locations, with the
exception of sites in arid or semiarid climates (e.g., certain
portions of the Basin and Range Province of the western U.S.).
Where the unsaturated zone is of a significant thickness greater
than tens of feet (e.g., 50 feet), its influence on time of
travel can be estimated by using the methods described in
Appendix C.
3.2 TIME OF TRAVEL (TOT) ANALYSIS
Time of travel (TOT) in a saturated, porous flow regime is
calculated from the following equation:
Time 8 Travel Distance (1)
Seepage Velocity
3-2
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Based upon the vulnerability criterion, Travel Distance is set at
100 feet. Fotf a site to have non-vulnerable hydrogeology, the
calculated TOT'must be on the oider of 100 years for waste
management units located over ground water. As the final factor
in the equation, the ground-water velocity term consequently
dictates whether a facility passes the TOT tests.
The velocity calculation uses information required in a
Part B permit application specific to ground-water flow; data
concerning waste constituent transport, such as dispersion,
are not required. The seepage velocity term used in the TOT
equation is an average linear velocity, V, derived from Darcy's
equation for saturated flow:
V = K I (2)
ne
where K is saturated hydraulic conductivity, I is the hydraulic
gradient (equal to the change in head divided by the length of
the flowpath (dh-/dl)), and ne is the effective porosity. The
average linear velocity, V, represents the rate of ground-water
flow through pore spaces only. It is indicative of the contam-
inant migration rate under the following conditions:
0 leaking waste constituents are miscible
with water,
0 ground-water flow occurs under saturated
conditions along a single flow line,
0 contaminants move advectively without
dispersion, and
0 contaminants are not retarded or degraded
within the ground water.
3-3
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/ - As shown in equation (2), V is proportional to K and I,
and inversely proportional to ne. Because this equation is a
linear relationship, the effect of parameter variability on V
is easy to determine. For example, if K or I is increased an
order of magnitude, V will increase one order of magnitude.
Conversely, a doubling of ne will decrease the calculated V
to one-half its original value. As this type of sensitivity
analysis demonstrates, accurate measurements of K, I, and ne
are required. Site-specific values for each of these parameters,
especially K and I, can only be obtained from a thorough hydro-
geologic investigation. The importance of each variable is
discussed in detail below.
t
3.2.1 Hydraulic Conductivity
Hydraulic conductivity (K) is a measure of the ease by
which a medium transmits ground water. It is one of the few
physical parameters that assumes values that may span more than
13 orders of magnitude. Although this entire range may not be
exhibited at a single site, hydrogeologic investigations often
reveal heterogeneity in K values that range over many orders of
magnitude. Aquifer heterogeneity may be related to either a
single stratum that is a geologically-complex depositional
environment (i.e., braided stream deposits or some glaciated
terrains), or a well-defined system of layered strata, such as
a sand-clay-sand sequence. Depending on test-well placement,
fractured bedrock typically exhibits extreme variability in
measured conductivities.
3-4
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To accurately characterize complex flow domains, an appro-
priate number of piezometers should be placed throughout
the saturated zone. Nested piezometers are used to determine
both K values Cor stratified deposits and the magnitude of the
vertical component of the gradient of flow. It is difficult
to specify a minimum number of piezometers and aquifer tests
that would be applicable for all potential HWLTSD sites. The
permit applicant and permit writer must determine the degree of
complexity of the saturated zone, using the permit applicant's
site characterization wnen evaluating the accuracy of reported K
values. Default values based on soil texture taken from the
literature and laboratory-derived values for K, instead of
>
field measured data, are not acceptable. Section 3.3 provides '
guidance on data reliability.
EPA has published proposed acceptable methods for determining
hydraulic conductivity in Test Method 9100: Methods for
Determining Saturated Hydraulic Conductivity and Saturated
Leachate Conductivity, for addition to the Agency's technical
guidance manual entitled, Test Methods for Evaluating Solid
Waste: Physical/Chemical Methods; OSW, SW-846. Appendix A
contains a condensed version of Method 9100 describing the field
methods that should be used to determine K.
Assuming that the site characterization provided in the
Part B permit application (see Section 2.1, Phase I Location
Guidance Manual, February 1985) provides an adequate basis for
calculating TOT, the next step is to assess the K value(s) used
in the calculation. Equation (2) assumes that for the represent-
ative flow line (or segment) taken from a flow net, K remains
3-5
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constant. In reality, this 100-foot flow line may traverse
several strata of varying conductivities. If the permit applicant
reports a range_ of K values along this flow path, the largest
hydraulic conductivity value should be used to simulate worst-case
conditions, unless the flow path can be divided into distinct
segments with unique values of K, I, and ne. If this is possible
(see Figure 3.2.1-1 for example), the individual travel times
of these segments can be summed.
Hydraulic conductivity may often be reported in several dif-
ferent units. Table 3.2-1 lists conversion factors for the
common units. Certain permit applicants may report transmissivity
instead of hydraulic conductivity. Transmissivity is derived
by multiplying hydraulic conductivity by the aquifer thickness *
and is expressed in units of length squared per unit time.
Therefore, hydraulic conductivity can be obtained by simply
dividing transmissivity by the aquifer thickness. However,
values of K derived from this manipulation of transmissivity
may not be reliable for the 100-foot flow line scale used in
this analysis. If the original value of transmissivity was
obtained by a large scale aquifer (or pump) test, using pumping
or observation wells that may have penetrated a number of
distinct geologic units, the resulting value of K will not be
characteristic of any one geologic unit. Values for K derived
from aquifer (or pump) tests are not acceptable for use in the
TOT analysis unless other subsurface investigations (e.g., test
borings with continuous flight sampling) show that the geoloc.c
materials over the interval in question are uniform.
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FIGURE 3.2.1-1: CROSS SECTION OF MULTIPLE GEOLOGIC STRATA
SHOWING SELECTED FLOW LINE BENEATH A
HAZARDOUS WASTE UNIT
HAZARDOUS WASTE
UNIT
-SELECTED FLOW LINE
GROUND WATER TABLE
\ /
:GEOLOGIC UNIT
1
2GEOLOGICAL UNIT
2
' '.'.«*.;"-"-."i..".'..-.'.«/ ':.ii*.' ''
vV..-;^/\-:/-V-:;;:v^;-'..-:*.y
A, B, and C are segments of the selected flow line.
A + B + C = 100 feet
Each geologic unit has different properties that determine
flow line direction.
3-7
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TABLE 3.2-1: HYDRAULIC CONDUCTIVITY CONVERSION FACTORS
Meters Per
Day
(ra/d-1)
1
8.64 x 102
3.05 x 10-1
4.1 x 10-2
Centimeters Per
Second
(CM/s-1)
1.16 x ID'3
1
3.53 x 10-4
4.73 x 10-5
Feet Per Day
(ft/d -1)
3.28
2.83 x 103
1
1.34 x 10-1
Gallons Per Day
Per Square Foot
(gas/d-l/ft-2)
2.45 x 10l
2.12 x 104
7.48
1
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3.2.2 Hydraulic Gradient
Hydraulic gradient (I)/ the driving force of ground-water
flow, is determined by dividing the change in head over the
distance between the points of measurement along the flow length.
Gradients can range from greater than 1.0 near a point of
ground-water discharge, to less than 0.0001, a value associated
with extensive areas of flat terrain. Factors influencing
hydraulic gradients include: (1) aquifer characteristics, (2)
conditions at the boundaries of the flow domain under consideration
(i.e., river elevation, ground-water discharge from the underlying
bedrock into a sand and gravel aquifer, etc.), and (3) system
inputs and outputs (i.e., rainfall, evapotranspiration, etc.).
»
System inputs/ outputs may include man-induced aquifer stresses-'
such as groundwater discharge or recharge at a well, or seepage
from a HWLTSD facility.
Selecting the initial value for head is a function of the
unit being examined. For surface impoundments, this initial
head is the maximum elevation of fluid within the impoundment
(see Figure 3.2.2-l(a)). For landfills, the initial head is the
maximum height of the saturated zone within the waste and cover
after unit closure, unless the fluid level can be higher prior
to this time (see Figure 3.2.2-Kb)). This height selection
should assume that any synthetic liner present in the unit will
eventually not function in the long-term and liner failure will
occur. For a waste pile, the initial head depends on whether
the unit has an approved liner as specified in the design and
operation standards under 40 CFR Part 264. For waste piles
3-9
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FIGURE 3.2.2-1: SELECTING INITIAL VALUES FOR HEAD AND THE
FLOW LINE
(a) Surface Impoundment
Fluid .
7s
\
X
(b) Landfill
,___ j__^ \
Waste
(c)(l) Waste Pile without liner
ll
(c)(2) Waste Pile with liner
Where h^ initial head and 1^ = initial point for flow line,
used in the expression for hydraulic gradient:
I - dn hl " h2
li - 12
where 1^ is the elevation of the end of the 100-foot flow line
and t\ is the head at l«
3-10
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without liners as specified in Part 264, the initial head is
the maximum height of the saturated zone within the waste in
the unit (see Eigure 3.2.2-1(c)(1) ). For waste piles with
approved liners, the initial head is located one foot above the
liner (see Figure 3.2.2-l(c)(2)). This height selection assumes
a properly functioning liner beneath the waste pile during the
short-term, active life of a storage unit from which waste will
be removed prior to closure. The initial points of the flow
lines for each unit are also shown on these figures.
Unlike hydraulic conductivity, I is susceptible to orders
of magnitude change due to aquifer stress, most often in the
form of aquifer exploitation as a water resource, but also due
t
to seasonal fluctuations. Therefore, for borderline sites, the
permit writer should carefully consider aquifer characteristics,
the areal extent of the aquifer, and the potential for future
ground-water use in the vicinity of the HWLTSD facility.
As mentioned in Section 3.Z.I, nested piezometers should
be used in site investigations to obtain information on the ver-
tical component of the hydraulic gradient. Further information
on the use of nested piezometers is presented in Appendix B.
Appendix A provides additional information on evaluating artificial
and seasonal influences on I, as well as methods for estimating
the maximum height of the saturated zone within a landfill with
no leachate collection system.
3.2.3 Effective Porosity
Effective Porosity (ne (%)) is the amount of interconnected
pore space in soils or sediments through which fluids can pass,
3-11
-------
expressed as a percent of bulk volume. Although effective poro-
sity is important in determining TOT, its impact on V is limited
to a much smaller range of potential values, unlike K and I. A
thorough site characterization should provide measured values
of ne. However, effective porosity data may not be available
for some sites; it can be estimated with little influence on
the validity of the TOT calculation. Table 3.2-2 gives default
estimates of effective porosity to be used when field data are
not available for calculating TOT. Where actual field data are
available, they should b« used instead of default values.
Effective porosity should not be confused with total poro-
sity, specific yield, or gravity drainage. Use of any of these
t
parameters as an estimate or substitute for effective porosity :
can affect resulting estimates of TOT by several orders of mag-
nitude in some cases. In some cases, such as for coarse grained
soils, the use of gravity drainable porosity or specific yield
may be acceptable. Appendix A presents an expanded discussion
of the relationship between effective porosity and other terms.
3.2.4 Necessary Precautions
In performing TOT analysis for determining ground-water
vulnerability, the permit writer must recognize that the rate
of travel and the initial appearance of hazardous wastes at the
100-foot end point are not being predicted. The permit applicant
and permit writer cannot predict when the engineered containment
structures will malfunction or, in many cases, what the concen-
trations of the released waste constituents will be at that
time. Darcy's law assumes that the constituents are traveling
advectively in an aqueous phase within the ground water.
3-12
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TABLE 3.2-2: DEFAULT VALUES FOR EFFECTIVE POROSITY FOR USE IN
TIME OF TRAVEL (TOT) ANALYSES
Effective Porosity
Soil Textural Classes of Saturationa
Unified Soil Classification System
GS, GP, GM, GC, SW, SPf SM, SC
ML, MH
CL, OL, CH, OH, PT
USDA Soil Textural Classes
Clays, silty clays, sandy clays
Silts, silt loams, silty clay loams
All others
Rock Units (all-)
Porous media (nonfractured rocks
such as sandstone and some carbonates)
Fractured rocks (most carbonates,
shales, granites, etc.)
0.20
(20%)
0.15
(15%)
0.01
(l%)b
0.01 *
(l%)b
0.10
(10%)
0.20
(20%)
0.15
(15%)
0.0001
(0.01%)
aThese values are estimates and there may be differences between
similar units. For example, recent studies indicate that
weathered and unweathered glacial till may have markedly
different effective porosities (Barari and Hedges, 1985; Bradbury
et al., 1935).
^Assumes de minimus secondary porosity. If fractures or soil
structure are present, effective porosity should be 0.001 (0.1%).
3-13
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Certain constituents in a ground-water flow regime can be
retarded to a velocity less than the average linear velocity due
to sorption or degradation. However, sorption and cation exchange
are reversible. In addition, every geologic media has a finite
retarding capacity. When the total mass and concentrations of
compounds released from the unit are unknown, one cannot know
if this capacity is adequate to stop or impede the migration
of the compounds. Mechanical dispersion is a transportation
mechanism that can reduce both the peak and steady-state concen-
trations of the contaminant and can also hasten the arrival or
the leading edge of the plume. However, dispersion rates are
generally unpredictable. \
If the permit writer encounters a case where such non-
vulnerable aquitards overlie, for example, a major high yielding
aquifer, then data should be examined, as feasible, to determine
the degree to which the 100-foot distance is representative of
the local ground-water flow system, and hence, of risk of exposure.
3.3 MODIFYING TOT^O FOR DATA RELIABILITY
TOT100 can be modified to reflect the reliability of the
values of the hydraulic parameters used in the calculation.
The modification is performed by multiplying TOT]_Q0 by a reli-
ability factor. Reliability can be judged on both the basis of
test statistics and the expected error of test procedures. For
example, hydraulic conductivity can be roughly estimated from
grain size distribution. However, the accuracy of this procedure
results in a two order of magnitude range when compared with
3-14
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Table 3.3-1: RELIABILITY FACTORS FOR ADJUSTING THE CALCULATION OF
Reliability Adjustment
General Reliability Class
Reliability Factor
Good
Fa ir
Poor
Comment
Hydraulic Conductivity (K)
Effective Porosity (rig)
.10
.01
If ne is not quantita-
tively determined, use
the default value from
Table 3.2-2.
Hydraulic Gradient (I)
.90
.50
3-15
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more precise, direct measurements. Consequently, EPA does not
consider it to be an acceptable method for calculating hydraulic
conductivity, as-discussed in Section 3.2.1. Table 3.3-1
presents reliability factors for adjusting the TOT calculation
for three classes of reliability for each of the three hydraulic
properties: hydraulic conductivity, hydraulic gradient, and
effective porosity. The reliability of an estimated parameter
is related both to the type and number of tests performed on
the strata of interest and to site-specific hydrogeologic con-
ditions at the location. The number of tests recommended are
based on professional judgement and field observation experience.
Although specific numbers are recommended, the appropriate
number of tests performed can be determined on a case-by-case
basis. The reliability of various test procedures, and the
recommended type and number of tests are listed in Table 3.3-2.
The TOT^QQ value should be multiplied by the appropriate reli-
ability value from the Table, after determining the reliability
class of each of the parameters. When the permit applicant
reports fewer tests than recommended for a particular parameter,
the reliability designation for that parameter estimate should
be reduced by one category. Examples of the use of reliability
factors to modify TOT are presented in the Technical Resource
Document entitled Technical Methods for Evaluating Facility
Location.
The permit writer must scrutinize the overall validity of
the calculated TOTj_oo before making the final determination of
vulnerability. TOT values in excess of 1000 years should be
3-16
-------
TABLE 3.3-2: RELIABILITY OF TESTS FOR DETERMINING HYDRAULIC FACTORS: HYDRAULIC CONDUCTIVITY, EFFECTIVE
POROSITY, AND HYDRAULIC GRADIENT'
Hydraulic Factor
Test Procedure*
INHERENT ERROR
RELIABILITY
COMMENT
Hydraulic Conductivity - Lateral
a. In-situ testing: 'slug test,'
auger hole method
Standard Lest
Good
Needs a minimum of five
slug tests or auger hole
method tests per strata.
I
f
*"*
b. Multiple aquifer tests*
c. Undisturbed core samples and
permeameter test
Standard test
Underest imates
by factor of
10 to 100
Fair to good
Fair to poor
Needs a minimum of ten sam-
ples per strata to account
for spatial variability.
d. Inference from grain size or
texture
May overestimate
or underestimate
by factor of 10
to 100
Poor
This procedure may be
fairly accurate tor coarse
textures, sands only.
e. Disturbed core samples and
permeameter
Underestimate by
factor of 100 to
10,000
Very poor
Unusable data.
Unacceptable method.
*See Method 9100 for specification of approved tests, in Test Methods for Evaluating Solid Kaste, Physical/
Chemical Methods, U.S. EPA, SW-846, April 19U4. '~rsf "
-------
TABLE 3.3-2: RELIABILITY OF TESTS FOR DETERMINING HYDRAULIC FACTORS: HYDRAULIC CONDUCTIVITY, EFFECTIVE
POROSITY, AND HYDRAULIC GRADIENT (Continued)
Hydraulic Factor-
Test Procedure*
INHERENT KRROR
RELIABILITY
COMMENT
Hydraulic Conductivity - Vertical
a. Undisturbed core samples and
permeameter
Standard test
Good
Needs a minimum of ten sam-
ples per strata to account
for spatial variability.
CO
I
H-*
00
b. Multiple aquifer tests
c. In-situ testing: 'slug test1
auger hole method
Standard test
Overest imates
by factor of
10 of 100
Fair to good
Fair to poor
Needs a minimum of five
slug tests or auger hold
method tests per strata.
Fairly precise in course
textures, sands, gravels.
Inference from grain size or
texture; Hazen Method
Hay overestimate
or underestimate
by factor of 10
Poor
This procedure may be
fairly accurate tor coarse
textures, sands, gravels.
Disturbed core samples and
permeameter
Undeiestimate by
factor of 100 to
10,000
Very poor
Unusable data.
Unacceptable method.
*See Method 9100 for specification of approved tests, in 'Jest 'Methods for Evaluating Solid Waste, Physical/
Chemical Methods, U.S. EPA, SW-846, April 1984.
-------
TABLE 3.3-2: RELIABILITY OF Tfc^TS FOR DETERMINING HYDRAULIC FACIURS: HYDRAULIC CONDUCTIVITY, EFFECTIVE
POROSITY, AND HYDRAULIC GRADIENT (Continued)
Hydraulic Factot-
Test Procedure
INHERENT ERROR
RELIABILITY
CO-WENT
Poiosity-Etfective
a. Undisturbed core, gravity
drain test
50%
Good
Standard test.
UJ
I
b. Undisturbed core, break-
through curve analyses
c. Estimate based on texture
and type of material
Unknown
100%
Good to fair
Fair
Specialized test for low
conductivity materials.
Hydraulic Gradient - Lateral
a. Water level - piezometric
isopleth map
10-20*
Good
Standard method.
b. 3-point method
20-50%
Fair
Common method for aqu it at-
testing.
c. Surface topography
Overestimates
up to 100%
Fair
Applicable to shallow sat-
urated zones only (within
50 feet of surface).
-------
TABLE 3.3-2: RELIABILITY OF TESTS FOR DETERMINING HYDRAULIC FACTORS: HYDRAULIC CONDUCTIVITY, EFFECTIVE
POROSITY, AND HYDRAULIC GKADlENf (Continued)
Hydraulic Factor
Test Procedure
INHERENT ERROR
RELIABILITY
COMMENT
Hydraulic Gradient - Vertical
a. Piezometer nests
10*
Good
Standard practice.
u>
I
N>
o
b. Assumption of 1.0
Unknown
Fair
Applicable to unsaturated
zone only. Gradients
>1.0 are possible in the
saturated zone.
Hydraulic Gradient - Change
Change due to changes in
surface hydrology or pumping
well installation
0-1000% plus
Poor
Gradients less than .10
are most susceptible to
man-induced changes.
-------
very rare and if they are reported, the permit writer should
double check each hydraulic component, including the flow path.
Strata with vertical hydraulic conductivities that are 1000 or
more times greater than lateral conductivities are rare and
should be closely evaluated. Differences in conductivity of
1000 or more between strata are uncommon and also need close
examination. Hydraulic gradients of less than 0.01 are extremely
shallow. In these cases, the gradient value used in the calcul-
ation should reflect the potential for gradient changes as a
result of pumping well installation or surface-water management
effects (i.e., water table drawdown from land drainage or water
table mounding from irrigation). Appendix A contains an
expanded discussion on gradient fluctuations.
3.4 USE OF ADDITIONAL CONTAINMENT STRUCTURES
In most cases, the influence of engineered structures
beyond the cap and synthetic liner (which are never factors in
the TOTj_oo calculation) should not be included in the calcula-
tion to adjust the TOT. As discussed in Section 4.0 and in the
report Investigation of Slurry Walls (OSW, June 1985), EPA will
consider such additional barriers as slurry walls and grout
curtains as enhancements to TOT only in certain hydrogeologic
settings. These settings should not be complex, and hydrologic
and geologic conditions at the location should be such that
they predominantly satisfy the vulnerable hydrogeology criteria.
Under certain conditions, these additional barriers could
particularly be considered for minimizing the potential for
exposure via the basement seepage pathway. These barriers can
3-21
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be
/*"
considered where minor sand lenses or soil fractures exist in
the shallow saturated zone, and where the criteria are othe-w
satisfied because the hydraulic conductivity of the predominant
native materials approach aquitard conditions that make placement
of production wells less llkeiy. Generally, there are no field-
supported data that demonstrates that the engineered structures
described above are effective in formation, that have overall
natxve permeabilities greater than lxlO'4 cm/second.
3-22
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4.0 APPLICATIONS OF ENGINEERED STRUCTURES
f "'
Modifying the design and operation of HWLTSD facilities as a
means of eliminating or minimizing the potential for exposure due
to migration of wastes may be appropriate in certain hydrogeologic
settings. Less vulnerable ground-water conditions can be created
in these settings through the use of additional passive contain-
ment barriers, such as slurry walls and grout curtains. These
additional engineering measures should only be considered in
non-complex settings that predominantly satisfy the tests for
ground-water vulnerability (i.e., test values are calculated to
be within twenty five percent of the reguired values). They
cannot be relied upon to make an obviously vulnerable site, such
as a sand and gravel aquifer, nonvulnerable. Section 4.3 describes
the types of hydrogeologic settings in which these methods can !:<=»
considered.
The term "engineered structures" does not refer to the
tures of the facility (such as liners and caps) that are cu»
reguired by RCRA regulations. It refers to additional act
passive control measures beyond current regulatory requir
that are installed at the facility. An active control
is an engineered feature that the owner or operator m>
or operate at his discretion to directly control the
volume, and direction of the ground-water flow syste
of an active control measure is a ground-water count
system that can be used to change flow direction. S
may be approoriate for lowering a water table, conta
plume, or collecting contaminated ground water for s>
4-1
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treatment. This technology is useful only as an immediate correc-
tive action measure to eliminate the potential for exposure due
to a waste release. It may not be a feasible perpetual care
measure because of the high cost of maintenance, operation, and
collection/ treatment facilities. Passive control measures are
engineered features used to control the ground-water flow direction
and velocity, but which are not "operated" or adjusted by the
owner or operator after the system is installed. Passive measures
include various types of ground-water flow barriers such as
slurry walls, grout curtains, and interceptor trenches. Grout
curtains and concrete slurry walls that are used for ground-water
containment may also serve as effective gas barriers in the
unsaturated zone.
The ensuing sections describe how certain passive engineered
structures may be used to both minimize the potential for exposure
at new and existing facilities and reduce the rate of waste migra-
tion to create less vulnerable ground-water conditions. Active
engineered structures are not considered to be appropriate means
to change a vulnerable setting into a nonvulnerable one.
4.1 SLURRY WALLS
Slurry walls and cutoff walls are subsurface barriers that
are emplaced to redirect or reduce ground-water flow. Slurry
trenching usually involves excavating a trench and backfilling it
to create a wall composed of soil-bentonite, bentonite-cement, or
an asphalt mixture which may be mixed with excavated soil. At a
HWLTSD facility, a slurry wall may be installed on the upgradient
side of the facility, forcing the ground water to flow around the
4-2
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wastes. A slurry wall may also be installed downgradient of a
site to divert ground-water flow, or installed to totally encircle
the facility and contain contaminated ground water beneath the
facility. These barrier walls/ once installed, must often be
accompanied by ground-water pumping systems to control hydraulic
gradients that may be altered by the wall installation.
If a slurry wall is to be installed at a site, the owner or
operator should submit detailed information regarding waste
compatibility; methods of excavation, keying the wall to bedrock,
and quality control; and possible changes in hydraulic gradients.
These factors will influence the performance of the slurry wall
in its ability to control ground water flow. A supporting document
entitled, Investigation of Slurry Walls, OSW, June 1985 (Draft)*,
contains information on the ability of slurry walls to minimize
contaminant migration and how these structures can be used most
effectively at RCRA facilities.
4.2 GROUT CURTAINS
Grout curtains are engineered structures used to control
ground-water flow and waste contaminant migration. More costly
than slurry walls, grout curtains are primarily used to seal
spaces in porous or fractured rock where other ground-water
controls would not be practical.
The method of installing a grout curtain is to drill holes
to the desired depth and inject the grout, under pressure, into
the voids. The grout may be one of two main types - suspension
grouts or chemical grouts.
4-3
-------
Grout curtains can be used to retard or reroute the flow
of ground water in porous rock. As with slurry walls, a grout
curtain can be jplaced upgradient from a waste site to redirect
ground-water flow. Problems may arise, however, in placing a
grout curtain downgradient from a site. It is possible that a
grout will lose its integrity in the presence of leachate or
contaminated ground water; extensive testing should be done to
assure that a grout would withstand extreme conditions at a
site and maintain its design integrity.
4.3 CONDITIONS APPROPRIATE FOR ADJUSTING TOT BASED UPON THE
PRESENCE OF ENGINEERED STRUCTURES
In certain hydrogeologic settings, passive measures to
control the ground-water flow systems at existing units may *
be effective in minimizing ground-water flow, thus decreasing
the vulnerability of the ground water. The effect of a passive
measure in changing the velocity of the ground-water flow system
should be considered when calculating the TOT. .The effect of
engineered barriers, for example, may greatly reduce the ground-
water flow velocity along each 100-foot segment of a flow path
beneath the facility. The barrier may also result in different
hydraulic gradients that must be considered in calculating TOT.
Passive engineered barriers and the extent to which these
control ground-water flow should only be considered in adjusting
the TOT in aquitard-like settings that predominantly satisfy the
TOT tests (i.e., within 25 percent) and that are not considered
complex. Accepting passive engineered barriers in hydrogeologic
settings that do not predominantly satisfy the TOT tests would
4-4
-------
clearly encourage the location of facilities in vulnerable
settings. This would be inconsistent with EPA's overall policy
for evaluating facility location, which is that the location
should serve to minimize the potential for exposure to wastes and
constituents released as a result of the failure of engineered
containment barriers.
Geologic complexity refers to the characteristics of geologic
stratification and structure. Geologically-simple locations are
typically characterized by a pancake-like arrangement of geologic
units having distinct boundaries that can be identified in the
subsurface investigation. Physical properties within each unit
vary little from one part to another and physical conditions
provide a stable setting. Locations become more complex when geo-
logic units are dipping or folded, when units end abruptly or are
discontinuous, when the boundaries become obscure, when physical
properties vary greatly within a layer (i.e., changes in perme-
ability values over several orders of magnitude / or when soil
conditions are unstable. The most complex sites are those where
information about geologic units and their physical properties
cannot be correlated based on boring data. In the worst case,
all subsurface features seem to be random, making predictions of
ground-water movement difficult or impossible. Terrains commonly
found to be geologically complex include the following:
0 Shallow bedrock areas composed of highly folded,
fractured, or faulted formations,
0 Karst areas,
0 Alluvial materials,
4-5
-------
0 Glaciated regions composed of fractured sediments,
and
0 Certain High Hazard and Unstable Terrains (see
Section.2.2 of the Phase I Location Guidance).
A location may predominantly satisfy the TOT tests in the
following way. For example, a site may be located in a geologic
setting composed primarily of a massive clay. However, thin
lenses of sand are found in some test borings. These sand lenses
do not appear to be continuous or interconnected. However, there
is a potential for ground water to flow over a part of the 100-
foot interval through one or more of these lenses. To minimize
the potential movement of contaminants through these sand lenses,
a slurry wall or grout curtain installed downgradient of the unit
»
may serve as a passive barrier that enhances the containment
property of the location.
When the slurry wall or grout curtain can be shown to have
acceptable design properties (e.g., resistant to degradation by
leachate), its effect on the TOT calculation can be considered as
any other geologic unit by adding the TOT through it to the TOT
calculated for the flow line on either side of the barrier. Of
course, the presence of this barrier may alter hydraulic gradients
at the site. Consequently, changes in these gradients as a result
of the barrier should be assessed.
4-6
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5.0 SUPPORTING DOCUMENTS AND REFERENCES
5.1 SUPPORTING DOCUMENTS
The Office of Solid Waste has analyzed a substantial number
of issues related to facility location during the past two years.
In addition to the Vulnerable Hydrogeology and Phase I location
guidance documents, the following reports and guidances have been
prepared or are in preparation to support the RCRA location program,
* Review of State Siting Criteria for Hazardous Waste Land
Treatment, Storage, and Disposal Facilities (January 1984)
This report describes various State regulatory approaches
and guidance manuals to control siting of hazardous waste
" '
facilities. State statutory requirements, regulatory stan-»
dards, technical guidance manuals, and State site selection
and mapping studies are described. - State siting criteria
can be divided into three main -ategocies.,, One set of
criteria define locations inadequate for siting of hazar-
dous waste management facilities. The second set defines
hydrologic agd* geolog|.iD ~faa«t,u-s*ers considered' .heee-ssary for
suitable sites. A third set includes setback criteria
defining separation distances between the facility and
off-site structures or geologic and hydrologic features.
e Permit Writers' Guidance Manual for Hazardous Waste Land
Treatment Units (Draft in preparation)
Land treatment units are subject to many of the same
location concerns as are storage^and disposal facilities,-
however, land treatment units are functionally different
from storage and disposal units and require a different
5-1
-------
approach in evaluating location. While storage and
disposal units are designed to contain hazardous consti-
tuents, land treatment units are sited and operated to
rely on natural soil conditions to degrade or transform
hazardous constituents. For this reason, a separate
location guidance manual for use by permit applicants
and writers in evaluating land treatment units is being
developed. The guidance focuses on methods to recognize
soil conditions that have a high probability of success
in meeting the treatment demonstration requirement for
land treatment unit permits.
* Data file of 225 Hazardous Waste Management Facilities,
Superfund Sites, and Site Enforcement Cases
This data file provides a variety of technical informa-
tion about facilities located in various hydrogeologic
settings. Each data file is being reviewed to provide
supporting information for development of future location
standards. Some of the facilities have been examined either
as location case studies included in the Technical Resource
Document entitled, Technical Methods for Evaluating Facility
Location, or in studies found in Appendix D of this guidance
0 Technical "White Paper" Series (February 1985; Draft Final)
The following reports have been developed on a variety
of technical issues related to facility siting and are
available to permit applicants and writers to serve as
background information:
- Use of Isotope Techniques in Estimating the Age and
Flow Direction of Ground Water at Hazardous Waste Sites
5-2
-------
- Characterization of Vertical Gradients and Impacts
in Siting Under Saturated Conditions
- Technical Issues Regarding Aquifers Containing
Variable Density Water and Their Effects on
Ground-Water Flow Systems
" - Technical Criteria for Defining Various
Locational Settings
* - Water Table Slope/Flow Gradient Relationship
- Techniques for Time of Travel (TOT) Calculations
- Review of RCRA 40 CFR Part 264.18 Location
Standards
° Results of Technical Peer-Review Committee Meeting for
Facility Locational Policy Development (Phase I)
(September 1984; Draft Final)
The objectives of the Committee were: 1) to identify
technical issues related to the siting of hazardous waste
management facilities, 2) to provide general comments on
the draft Phase I Location Guidance Manual and suggestions
for corrections or addition of material to the document,
and 3) to present case study examples illustrating various
technical issues related to location.
0 Investigation of Slurry Walls, OSW, June 1985 (Draft
Supporting Document)
This report examines the ability of slurry walls to
minimize contaminant migration in order to assess how
they could be used at RCRA facilities. Reviewers of early
drafts of the vulnerable hydrogeology guidance manual
raised several questions on the use of manmade engineered
structures, such as slurry walls. One major question
focused on the amount of credit, if any, that a facility
should be given if a slurry wall already exists or is
5-3
-------
proposed as a means of containing contaminants. Also, ifc a
slurry wall is given credit, should it be factored into calcu-
lating the contaminant TOT?
° Regulatory Development Plan Location Standards for RCRA
Hazardous Waste Facilities, Office of Solid Waste, U.S.
Environmental Protection Agency, December 1985.
° Report on the Review_of the "Permit Writers' Guidance Manual
for the Location of Hazardous Waste Land Treatment, Storage, and
Disposal Facilities Phase II", Environmental Engineering Committee,
Science Advisory Board, U.S. Environmental Protection Agency,
June 1986.
5-4
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