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

-------
             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

-------
                              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
                                                                >
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.

-------
                         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.

-------
                        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
                                                               t
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.

-------
  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

-------
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

-------
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
                                                                 >
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

-------
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

-------
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

-------
 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

-------
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

-------
                             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

-------
                                                             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

-------
                             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

-------
                         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

-------
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
                               1-2

-------
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
                               1-3

-------
         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
                                         1-4

-------
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
                                                                t
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
                               1-5

-------
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

-------
to verify hydrogeologic information provided in the application.2/



     Where results of the method indicate that a facility is
               r"


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.
                               1-7

-------
                    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






                               2-1

-------
           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.

                                          2-2

-------
     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

-------
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

-------
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

-------
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

-------
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

-------
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

-------
          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

-------
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

-------
     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

-------
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

-------
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

-------
 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

-------
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

-------
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

-------
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

-------
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

-------
/• -               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

-------
     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

-------
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.
                               3-6

-------
     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

-------
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
                              3-8

-------
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

-------
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

-------
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

-------
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

-------
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

-------
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

-------
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

-------
         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

-------
                      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

-------
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

-------
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

-------
           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

-------