&EPA
United Slates
Environmental Protection
Agency
Office of
Soiid Waste and
Emergency Response
Publication 9234.2-25
EPA,'S40-R-93-GBO
PB93-963507
September 1993
Superfund
Guidance for Evaluating the
Technical Impracticability of
Ground-Water Restoration
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Directive 9234,2-25
September 1993
Guidance for Evaluating the
Technical Impracticability of
Ground-Water Restoration
Interim Final
Office of Solid Waste and Emergency Response
U.S. Environmental Protection Agency
Washington, DC 20460
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1.0 Introduction
1.1 Background
Restoration3 of contaminated ground waters is one of
the primary objectives of both the Superfund and
RCRA Corrective Action programs. Ground-water
contamination problems are pervasive in boih pro-
grams; over 85 percent of Superfund National Priori-
ties List (NPL) sites and a substantial portion of
RCRA facilities have some degree of ground-water
contamination. The Superfund and RCRA Corrective
Action programs share the common purposes of pro-
tecting human health and the environment from con-
taminated ground waters and restoring those waters
to a quality consistent with their current, or reason-
ably expected future, uses.
The National Contingency Plan (NCP). which pro-
vides the regulatory framework For ihe Superfund
program, states that:
"EPA expects to return usable ground waters to
their beneficial uses wherever practicable,
within a timefrarne that is reasonable given the
particular circumstances of the site"
(NCP §300.430(a){l){iiiXF)).
Generally, restoration cleanup levels in the Superfund
program are established by applicable or relevant and
appropriate requirements (ARARs), such as the use of
Federal or Suite standards for drinking water quality.
Cleanup levels protective of human health and the en-
vironment are identified by EPA where- no ARARs for
particular contaminants exist (.see Section 4,1,1).
The RCRA Corrective Action program for releases
from solid waste management facilities (see 40 CFR
264.10t}3 requires a facility owner/operator to:
"...institute corrective action as necessary to pro-
tect human health and the environment for all
releases of hazardous waste or coasu'iuenis from
any solid waste management unit..."
The goal of protecuveness is further clarified in the
Preamble to the Proposed Subpari S to 40 CFR 264:
"Potentially drinkable ground water would be
cleaned up to levels safe for drinking throughout
the contaminated plume, regardless of whether the
water was in fact being consumed.,. Alternative
levels protective of the environment and safe for
other uses could be established for ground water
that is not an actual or reasonably ex pec-ted source
of drinking water."-*
While both programs have had a great deal of success
reducing the immediate threats posed by contami-
nated ground waters, experience over the past decade
has shown that restoration to drinking water quality
(or more stringent levels where required) may not al-
ways be achievable due to the limitations of available
remediation technologies (EPA !989b, I992d). EPA,
therefore, must evaluate whether ground-water resto-
ration at Superfund and RCRA ground-water cleanup
sies is attainable from an engineering perspective,
This document outlines EPA's approach to evalu-
ating the technical impracticability of attaining re-
quired ground-water cleanup levels and establish-
ing alternative, protective remedial strategies
where restoration is determined to be technically
impracticable,
Many factors can inhibit ground-water restoration,
These factors may be grouped under ihree general
categories:
» Hydrogeologie factors;
• Contaminant-related factors; and
• Remediation system design inadequacies.
Hydrogeologie lirniialions to aquifer remediation in-
clude conditions such as complex sedimentary depos-
its; aquifers of very low permeability; certain types of
1 For this guidance, "restoration" refers to the reduction of contaminant goncensratiuns to levels required under the Supeil'und
or RCRA Corrective Action programs. For ground water currently or potentially used for drinking water purposes, these lev-
els may b« Maximum Contaminant Levels (MCLs) or non-zero Maximum Contaminant Levels Goals. (MCljGs) teiaMwhwJ
under the Safe Drinking Water Act: Stale MCLs or oEhar cleanup requirements; o? risk-based levels fix compounds not cov-
ered by specific State or Federal MCLs or MCLGs. Other cleanup levels may be appropriate for ground waters used for mm-
drinkm| water purposes.
2 At this time, this guidance is not applicable to corrective actions for releases from Subjwi F regulated units that are subject to
corrective actions under 40 CFR 264.91-264.100,
3 "Corrective Action for Solid Waste Management Units; (SWMUsJ at Hazardous Waste Management Facilities." 55 ££. 36798-
30884, July 27, 1990, Proposed Rules, is currently used as guidance in the RCRA Currcctive Action program. When final
regulations under Subpaii S are promulgated, cenain aspects of this guidance pertaining w the RCRA program may need to be
revised to reflect new regulatory requirements.
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fractured bedrock; and other conditions that presently
make extraction or in siiu treatment of contaminated
ground water extremely difficult. (Figure 1),
Contaminant-related factors, while not independent
of hydrogeologic constraints, are more directly re-
lated to contaminant properties that may limit the
success of an extraction or in situ treatment process.
These properties include a contaminant's potential 10
become either sorbcd onto, or lodged within, the soil
or rock comprising the aquifer, Nonaqueous phase
liquids (NAPLs) arc examples of contaminants that
may pose such technical limitations to aquifer resto-
ralion efforts. NAPLs that are denser than water
{DNAPLs) often are particularly difficult to locate
and remove from the .subsurface; their ability to sink
through the water table and penetrate deeper portions
of aquifers is one of the projjenies thai makes ihem
very difficult to remediate (Figure 1),
The widespread use of DNAPLs in manufacturing
and many other sectors of ilic economy prior to the
advent of safe waste-management practices has ted to
their similarly widespread occurrence at ground-wa-
ter contamination sites. Most of the sites where EPA
already has determined that ground-water restoration
is technically irnpracticable have DNAPLs present,
The potential impact of DiNAPL contamination on at-
tainment of remediation goals is so significant that
EPA is developing specific recommendations for
DN APL site management; the key elements of this
strategy are presented in Section 3.0 below.
The third factor that may limit ground-water restoration
is inadequate remediation system design and imple-
mentation. Example* of design inadequacies in a
ground-water extraction system include an insufficient
number of extraction points (e.g., ground water or va-
por extraction wcifs) or wells whose locations,
screened intervals, or pumping raies lead lo an inability
so capture, the plume. Design inadequacies may result
from incomplete site clmracteri/ijiion, such as inaccu-
rate measurement of hydraulic conductivity of the af-
fected aquifer or not considering the presence of NAPL
contamination. Poor remediation system operation,
such as excessive downtime or failure to modify or
enhance the system to improve performance, also
may limit the effectiveness, of restoration efforts.
failure to achieve desired cleanup standards re-
sulting from inadequate system design or opera-
tion is not considered by EPA to be a sufficient
justification for a determination of technical im-
practicability of ground-water cleanup.
1.2 Purpose of the Guidance
Tins guidance clarifies how EPA will determine
whether ground-water restoration is technically im-
practicable and what alternative measures or actions
must be undertaken to ensure that the final remedy is
protective of human health and die environment.
Topics covered include the types of technical data
and analyses needed to support EPA's evaluation of a
particular sice and the criteria used to make a determi-
nation. As technical impracticability (Tl) decisions are-
part of the process of site investigation, remedy selec-
tion, remedial action, and evaluation of remedy perfor-
mance, the guidance aiso briefly discusses she overall
framework for decision making during these phases of
site cleanup,
This guidance does not signal a scaling back of
EPA's efforts to restore contaminated ground wa-
ters sit Superfund sites and RCRA facilities.
Rather, EPA is promoting the careful and realistic as-
sessment of the technical capabilities ai hand to man-
age risks poned by ground-water contamination. This
guidance provides consistent guidelines for evaluat-
ing technical irapracticability and tor maintaining
prorecuveness at sites where ground water cannot be
restored within a reasonable timeirarne. EPA will
continue to conduct, fund, and encourage research
and development in the fields of subsurface assess-
mem, remediation, and pollution prevention so that
an ever decreasing number of sites will re-quire the
analysis described in Oils document,
2.0 Ground-Water Remedy
Decision Framework
2,1 Use of the Phased Approach
At sites with very complex ground-water contamina-
tion problems, is may be difficult to determine
whether required cleanup levels are achievable ai the
time a remedy selection decision must be made. This
is especially true when such decisions most be based
on sue data collected prior to implementation and
monitoring of pilot or full-scale remediation systems.
EPA recognizes this limitation and has recommended
several approaches to reduce uncertainty during the
site eharacteri/auon, remedy selection, anci remedy
implementation processes (EPA I989a, 1992a).
Determining the restoration potential of a site may he
aided by employing a phased approach to site char-
acterization and remediation. Each phase of site
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Figure 1, Examples of Factors Affecting Ground-Water Restoration
Certain site characteristics may limit the effectiveness of subsurface remediation. The examples listed below arc
highly generalized. The particular factor or combination of factors that may critically limit restoration potential
will be site specific.
Generalized Remediation Difficulty Seal©
Contaminant Increasing difficulty
Characteristics
5
Nature of Release
Small Volume
Short Dura lion
Slug Release
Large Volume
Long Duration
Continual Release
Chemical Properties
Biotic/ Abiotic Decay
Potential
Volatility
Contaminant
Retardation (Sorption)
Potential
Hsgli ~ LOA
T ,,... - fer Hi"b
Contaminant Phase
Volume of
Contaminated Media
Contaminant Depth
Aqueous, Gaseous — * Sorbed — +• LNAPLs • — > DNAPLs
-*• Large
Hydrogeoioglc
Characteristics
Stratigraphy
Texture of
Unconsolidated Deposits
Degree of Heterogeneity
Simple Geology, —
e.g., Planar Bedding
-»• Complex Geology,
e.g., 1 nterfaeekled and Discontinuous SLrata
Homogeneous
(e.g., well-sorted sand)
Heterogeneous (e.g., mierbedded sand and
silts, clays, fractured media, karst)
to
s*.
Hydraulic conductivity
Temporal Variation
Vertical Flow
High O10-1 cm/sec)
Low (< 10" cm/sec)
High
. Flow Component
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characterkalion should be designed 10 provide infor-
mation necessary for the next phase of characteriza-
tion. Likewise, site remediation activities can be con-
ducted in phases to achieve interim goals at the oul-
sei, while developing a more accurate understanding
of the restoration potential of the contaminated aqui-
fer. An example of how this approach might be. ap-
plied at a site is provided below in Section 4,4.3.
The liming ol phased cleanup actions (early, interim,
final) should reflect the relative urgency of ihe action
and die degree to which the site has been character-
ized. Early actions should focus ois reducing the risk
posed by site contamination (e.g., removal of con-
tamination sources) and may be carried out before de-
tailed site characterization studies have been com-
pleted. Interim remedial actions may abate the
spread of contamination or limit exposure but do not
fully address the final cleanup levels for the site. In-
terim actions generally will require a greater degree
of site characterization than early actions. However,
implementation of interim actions still may be appro-
priate prior to completion of site characterization
studies, such as ihe- Remedial Investigation/Feasibil-
ity Study (RI/FS) or RCRA Facility Investigation
(RFl) and Correciive Measures Study (CMS). Final
remedial actions must address the cleanup levels and
other remediation requirements for the site and, there-
fore, musi be based on completed characterisation re-
ports. Information from early and interim actions
aJso should be factored into these reports and final
remedy decisions,
Phasing of activities generally should not delay or
prolong site characterization or remediation. En fact,
such an approach may accelerate the implementation
of interim risk reduction action and lead more
quickly to the development of achievable final reme-
diation levels and strategics. A phased approach
should be considered when there is uncertainty re-
garding the ultimate restoration potential of Use sits
but also a need to quickly control risk of exposure to,
or limit further migration of, the contamination.
It is critical thai the performance of phased remedial
actions (e.g., control of plume migration) be monitored
carefully as pan of the ongoing effort to enaraeieri/e
the site and assess its restoration potential. Data collec-
tion activities during such actions not only should be
designed 10 evaluate performance with respect to the
action's specific objectives but also contribute to she
overall understanding oi' ihe site. In this manner,
actions implemented earl)1 in die site remediation
process can achieve significant risk redaction and
lead 10 development of technically sound, final rem-
edy decisions.
2.2 Documenting Ground-Water Remedy
Decisions Under CERCLA
ITie phased approach to site characteri/alion and
remediation can be employed using ihe existing deci-
sion document options within ihe Superfimd pro grant,
2,2,1 Removal Actions
Removal authority can lie used fur early actions as
pan uf a phased approach :o ground-water cleanup
and decision making and should be considered
where early response to ground-water contamination
is advantageous or necessary. Wilhin the contest ol"
ground-water actions, removals are appropriate
where contamination poses an actual or potential
threat to drinking water supplies or threatens sensi-
tive ecosystems. Examples of actions that might
qualify Tor use o>f removal authority include removal
of surface sources (e.g., drums yr highly contami-
nated soils), removal of subsurface sources (C.K-,
NAP!, accumulations, highly contaminated soils, or
other buried waste), und containment i>f migrating
ground-wafer contamination "hot spots" (?ones of
high contaminant cone en try Lion ;• or plumes to protect
current or (Njieniiu) drinking waier supplier.
Removals of subsurface sources most likely will be
non-umc-eritieal actions, although time-critical ac-
tions may be appropriate for removal of NAPL ac-
cumulations or other sources, depending on ihe ur-
gency of the threat. Documentation requirements
for removal actions include a Removal Action
Memorandum and, tor non-time critical actions, an
Engineering Evaluation/Cost Analysis report4
Removal actions must attain ARARs to the extent
practicable, considering the exigencies of the
situation. The urgency of the situation and die seojw
of the removal action may be considered when
determining the practicability of attaining ARARs
(NCP $300.415(i)J. Standards or regulations typically
used to establish ground-water cleanup levels for final
actions (e,g,: MCLs/MCLGs) may not Ix- ARARx,
depending on the scope ol' the removal, Further
4 Se« "Guidance on Conducting Nun Time Critical Removal Action's,
August 1993 (BPA I993bj,
kr CERCLA," OS WER Publication 93611.0-32,
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inlormtttion on removal actions may be found in
other EPA guidances (EPA Iy90b, 199id).
2,2,2 Interim ttODs
Interim RODs may be appropriate where there is a
moderate to high degree of uncertainly regarding at-
tainment of ARARs or oilier protective cleanup lev-
els. As mentioned before, an interim action may be
used to minirm/c further contaminant migration and
reduce the risk of exposure to contaminated ground
water. Interim actions include containment of the
leading edge of a plume to prevent further contami-
nation of unaffected portions of an aquifer, removal
of source material, remediation of ground-water hot
spots, and in some cases, installation of physical
barriers or caps u> contain releases from source ma-
terials. Interim actions should bt- monitored care-
fully to collect detailed information regarding aqui-
fer response 10 remediation, which should he used to
augment and update previous siie characterization
efforts. This information then can IK used at a lauir
date to develop final remediation goals and cleanup
levels that more accurately reflect the particular con*
diuons of the site.
It is important to note that for interim actions,
ARARs must be attained only if they arc. within the
scops ot thai action. For example, where an interim
action will manage or contain migration of an aque-
ous contaminant plume-, MCLs and MCLGs would
not be ARARs, since the objective of the action is
containment, not cleanup (although requirements
such as those related to discharge of the treated water
still would be ARARs, since they address the disposi-
tion of treated waste),
Furthermore, a requirement dial is art ARAR for an
interim action may be waived under certain circum-
sianccs. An "'interim action" ARAR waiver may be
invoked where an interim action drat does not attain
an ARAR is part of. or will be followed by, a final
action that does (NCP §3 00,430(0(1 }{ii)(C)). For ex-
ample, where an interim action seeks to reduce con-
tamination levels in a ground-water hot spot, MCLs/
MCLGs may be ARARs since the action is cleaning
up a portion of lite contaminated ground water. If,
however, this interim action is expected to be fol-
lowed by a final, ARAR-c-ompliant action that ad-
dresses the entice contaminated ground-water zone,
an interim action ARAR waiver may be invoked.
23.3 Final RODs
Where site characterization is very thorough and
there is a moderate to high degree of certainty ibat
cleanup levels can be achieved, a final decision docu-
ment should be developed that adopts those levels,
Conversely, in cases where there is a high degree of
certainly that cleanup levels cannot be- achieved, a final
ROD that invokes a TI ARAR waiver and e-stabiishes
an alternative remedial strategy may be ibe most appro-
priate option,5 Note that for ROD-siage waivers, site
characterization generally should be sufftciendy de-
tailed to address die data and analysis requirements for
Tl determinations set forth in this guidance,
2,2,4 ROD Contingency Remedies and
Contingency Language
Where a moderate degree of uncertainty exists re-
garding the ability to achieve cleanup levels, a final
ARAR-compliant ROD generally still is appropriate,
However, the ROD may include contingency lan-
guage that addresses actions to be taken in the event
the selected remedy is unable 10 achieve the required
cleanup levels (EPA I990a, 199la). The contingency
language may include requirements to enhance or
augment the planned remediation system as well as
an alternative remedial technology to be employed if
modifications to the planned system fail to signifi-
caiiUy improve its performance. Use of language in
final remedy decision documents that addresses the
uncertainty in achieving required cleanup levels also
is appropriate in certain cases. However, language
that identifies u Tl decision (e.g., an ARAR
waiver) as a future contingency uf the remedy
should be avoided. Such language is not necessary,
as a TI evaluation may be performed (and a decision
made) by EPA at any site regardless of" whither such
a contingency is provided in the decision document.
Note that in rases of existing RODs that ulresidy
include a contingency for invoking a TI ARAR
waiver, the conditions under which tht ARAR
may be waived shuuld be consistent with, and as
stringent us, those presented in this guidance or a
future update.
Furthermore, the fact that such contingency Ian-
guage has been included in an existing ROD dots
not alter the need to enhance or augment a rem-
edy to improve its ability to attain ARARs before
concluding that a waiver can be granted. H also
5 At sices wht-rc a "['[ ARAR waive! is invoked in the ROD, preparaucm of ihe pre-icIXTrai negotiation package ("mini-lit" pack-
ige) must intludu jincilysis ol the model Con&L'iu Degree language to ensure umi iippryprisiie consideration of ihc waiver's im-
part is incorporate.!!.
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should be noted thai remediation must be conducted
for a sufficient period of lime before its ability to re-
store contaminated ground water can be evaluated.
This minimum time period will be determined by
EPA on a site-specific basis,
2.3 Documenting Ground-Water Remedy
Decisions under RCRA
The instruments used for implementing the RCRA
Corrective Action program (permits and orders) also
are amenable to a phased approach to remedy selec-
tion and facility remediation, Hie RCRA program
tan use permits or orders to compel both interim
measures and final remedies.
23.1, Permits/Orders Addressing Stabilization
RCRA permits or orders can require the stabilization
of releases from solid waste management units
(SWMUs) at tiie facility. The Stabilization initiative
focuses on taking interim actions to prevent the fur-
ther spread of existing contamination and reduce
risks. Examples of measures used for stabilization
include capping, excavation, and plume containment.
Since the long-term or final cleanup of the facility is
not the objective of stabilisation (although stabiliza-
tion should be consistent with the final remedy), TI
decisions are not applicable at this early stage, Infor-
mation gained during stabilization should be used to
help determine the restoration potential of the facility
and the objectives of the final remedy.
233. PermtislQrders Addressing Final Kemvdies
Where achieving ground-water cleanup standards is
determined by EPA to be technically impracticable,
the pennii or order addressing final remedies should
include practicable and protective alternative reme-
dial measures. EPA's decision to make a Tl determi-
nation will be based on clear and convincing infor-
mation provided by the owner/operator. EPA gener-
ally will seek public comment on TI determinations
prior to implementation, EPA's preliminary Tl deter-
minations and justification for these determinations
should be documented in a Statement of Basis. As
discussed above, uncertainty in the ability to restore
an aquifer should be reduced through phased charac-
terization and the use of interim remedial measures,
where appropriate.
Permits and orders that address "fired" remedies should
specify the remediation cleanup levels selected by the
implementing Agency. Such permits and orders, how-
ever, generally should not incorporate contingency Tl
language. The permit, or order will need to be modified
to document the Tl determination and 10 specify, as
appropriate, alternative cleanup levels and alternative
remedial measures thai have been determined to be
technically practicable and protective of human health
and the environment,
3.0 Remedial Strategy for
DNAPL Sites
Many of the subsurface contaminants present at Su-
perfund sites and RCRA facilities are organic com-
pounds that are either Hghter-than-waier N APLs
(LNAPLs) or DNAPLs, As mentioned in Section 1,1,
the presence of N APL contamination, and in particu-
lar DNAPL contamination, may have a significant
impact on site investigations and die ability to restore
contaminated portions of ihe subsurface la required
cleanup levels. Furthermore, DNAPL contamination
may be. a relatively widespread problem. A recent
EPA study (EPA 1993 a) concluded that up 10 60 per-
cent of National Priorities List (NPL) sites may have
DNAPl, contamination in the subsurface; a signifi-
cant percentage of RCRA Corrective Action facilities
aiso are thought 10 be affected by DNAPLs, As
proven technologies for the removal of certain types
of DNAPL contamination do not exist yet, DNAPL
sites are more likely to require. TI evaluations than
sites with other types of contamination. Although
this guidance pertains to Tl evaluations at all site
types. EPA believes tlie significance of the DNAPL
contamination problem warrants the following brief
discussion of DNAPL contamination and recom-
mended site management strategies.
DNAPLs comprise a broad class of compounds, in-
cluding creosote and coal tars, polychlorinaied bipbe-
nyls (PCBs), certain pesticides, and chlorinated or-
ganic solvents such as inchloroelhylene (TCE) and
leirachloroethySenc (PCE), The term "DNAPL" re-
fers only to liquids immiscible in, and denser than,
water and not 10 chemicals thai are dissolved in water
ihas originally may have been derived from a DNAPL
source. DNAPLs may occur as "free-phase" or "re-
siduai" contamination. Free-phase DNAPL is an im-
miscible liquid in the subsurface that is under positive
pressure; that is, the DNAPL is capable of flowing
into a well or migrating laterally or vertically through
an aquifer. Where vertically migrating free-phase
DNAPL encounters a rock or soil layer of relatively
low permeability (e.g., clay or oilier fine-grained layer),
a DNAPL accumulation or "pool" may form. Residual
DNAPL is immiscible liquid held by capillary forces
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within ihe pores or fractures in soil or rock layers;
residual DNAPL, therefore, generally is not capable
of migrating or being displaced by normal ground-
water flow. Both free-phase and residual DNAPL,
however, can slowly dissolve in ground water and
produce "plumes" of aqueous-phase contamination,
DMAPLs also can produce subsurface vapors capable
of migrating through the unsaturated zone and con-
taminating ground water (EPA I992c). Figure 2 dc-
picis the various types of contamination thai may be
encountered at a DNAPL site.
"The three ureas thai should be delineated at a
DNAPL site are ihe DNAPL entry location, ihe
DNAPL zone, and ihe aqueous contaminant plume,
The entry locations are those areas where DNAPL
was released and likely is present in the subsurface.
Envy locations include waste disposal lagoons, drum
burial sites, or any other area where DNAPL, was al-
lowed to infiltrate- into the subsurface. The DNAPL
zone is defined by that portion of the subsurface con-
taining free-phase or residual DNAPL, Thus, the
DNAPL zone includes ail portions of the subsurface
where the immiscible-phase contamination has come
to be located, The DNAPL zone- may occur within
both the saturated zone (below the water table) and
ihe unsaturated .tone (above the water table). The
DNAPL /.tine also may contain vapor and aqueous-
phase- contamination derived from the DNAPL. The
DNAPL /one may include areas at relatively great
depths and lateral distances from the entry locations,
depending on the subsurface geology and the volume
of DNAPL released. The aqueous contaminant
plume contains organic: chemicals in ihe dissolved
phase-. The plume originates from ihe DNAPL zone
and may extend hundreds or thousands of feet
downgradicm (in ihe direction of ground-water flow),
Figure 3 illustrates the various components of a
DNAPL site,
Sinc-e each DNAPL site component may require a
different remediation strategy, it is important 10 char-
acterize these components to the extent practicable.
Thus, the properties and behavior of DNAPL con-
tamination require consideration when planning and
conducting both site investigation and remediation.
The potential for DNAPL occurrence at the site
should be evaluated as early as possible in the site in-
vestigation, Recent publications such as "Estimating
Potential for DNAPL Occurrence at Superfuncl Sites"
(EPA !992c) and "DNAPL Siie Evaluation" (Cohen
and Mercer, 1993) provide detailed guidance on
these- topics, At sites where DNAPL disposal is
known or suspected to have, occurred, likely DNAPL
entry locations should be identified from available
historical wasie-managemem information arid sub-
surface chemistry data. This information can assist
in the delineation of ihe DNAPL zone.
Characterization and delineation of ihe DNAPL none
is critical for remedy design and evaluation of the
restoration potential of the site, Ai many sites, a sub-
surface investigation strategy that begins outside of
ilie suspected DNAPL /.one may be appropriate
("oui.side-in" strategy), in part 10 minimize the possi-
biliiy of inadvertent mobilization of DNAPLs to
Figure 2. Types of Contamination and Contaminant Zones at
DNAPL Sites (Cross-sectional view)
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Figure 3. Components of DNAPL Srtes
DNAPU 2om
contains (fa*-pfta*e DMA PL In pass
or lenses and'o? 'esloual ONAPL
ONAPL Efflry Location
' as a farmer waste DOf-d
lower aquifers. Delineation of ihc extent of the
DNAPL zone may be difficult at certain sites due to
complex geology or waste disposal practices. In such
cases, the extent of the DNAPL zone may need to be
inferred from geologic information (e.g., thickness,
extent, structure, and permeability of soil or rock
units) or from interpretation of the aqueous concen-
tration of contaminants derived from DHAPL
sources. Ai some sites, however, geologic complex-
ity and inadequate information on waste disposal may
make the delineation of the DNAPL zone difficult,
A phased approach, as discussed in Section 2,1, is
recommended for DNAPL siics; such an approach
may facilitate identification of appropriate short- and
long-term site remediation objectives. Note also thai
technical approaches appropriate for the DNAPL
zone (e.g., free-phase DNAPL removal, vapor extrac-
tion, excavation, and slurry walls aided by limited
pump-and-treat) may differ significantly from those
appropriate for the aqueous contaminant plume (typi-
cally pump-and-tieal).
Short-term remediation objectives generally should
include prevention of exposure 10 contaminated
ground water and containment of the aqueous con-
taminant plume. Where sufficient information is
available, early removal of DNAPL sources also is
recommended. Information gathered during these
actions should be used to help characterize the site and
identify practicable options for further remediation.
The Song-term remediation objectives for a DNAPL
zone should be to remove the Free-phase, residual,
and vapor phase DNAPL to the extent practicable and
contain DNAPL sources dial cannot be- removed.
EPA recognizes thai if may be difficult to locate and
remove all of the subsurface DNAPL within a
DNAPL zone. Removal of DNAPL mass should be
pursued wherever practicable and, in general, where
significant reduction of current or future risk will re-
sult.6 Where u is technically impracticable to remove
subsurface DNAPLs, EPA expects lo contain the
DNAPL zone ED minimize further release of contami-
nants to the surrounding ground water, wherever
practicable,'
Where it is technically practicable to contain ihe
long-term sources of contamination, such as the
DNAPL zone, EPA expects to restore ihe aqueous
contaminant plume outside the DNAPL zone to re-
quired cleanup levels. Effective containment of ihe
DNAPL zone generally will be required to achieve
this long-term objective because ground-water ex-
traction remedies (e.g., pump-and-ueat) or 1/2 situ
treatment technologies are effective for plume resto-
ration only where source areas have been contained
or removed,
DNAPL mass removal also must satisfy ite Suped'urei or RCRA Corrective Action remedy selection criteria, as appropriate.
As DNAPLs may be remobilized during drilling or ground-water pumping, cauuon should be exercised when: such activities
rue proposed for DNAPL zone characterization, remediuiiun, or «>nummcni.
-------
Monitoring and assessing the performance of
DNAPL zone containment and aquifer restoration
systems, therefore, are critical to maintaining remedy
protectiveness and evaluating the need for remedy
enhancements or application of new technologies.
EPA recognizes, however, that there are technical
limitations to ground-water remediation technologies
unrelated to the presence of a DNAPL source /.one.
These limitations, which include contaminant-related
factors (e.g., stow desorption of contaminants from
aquifer materials) and hydrogeologic factors (e.g.,
heterogeneity of soil or rock properties), should be
considered when evaluating the technical practicabil-
ity of restoring the aqueous plume,
EPA encourages consideration of innovative technolo-
gies at DNAPL sites, particularly where containment
of a DNAPL zone may require costly periodic mainte-
nance (and perhaps replacement). Innovative technolo-
gies, therefore, should be considered where DNAPL
zone containment could be enhanced or where such a
technology could clean up ihe DNAPL zone,
4.0 Tl Decisions and Supporting
Information
4.1 Regulatory Framework for TI Decisions
The bases for Tl decisions discussed in this guidance
are provided in CERCLA and the NCP for the Super-
fund program and in the Proposed Subpart S rule for
the RCRA program. While Lite processes the two pro-
grams use to establish cleanup levels differ (e.g., ihe
ARAR concept Is not used in RCRA}, the primary con-
siderations for determining the technical impracticabil-
ity of achieving those- levels are identical:
• Engineering feasibility; and
• Reliability.
A brief summary of the regulatory basis for establish-
ing cleanup levels and making Tl determinations at
Superfund and RCRA sites is provided below.
4.1.1 Supe rfund
Remedial alternatives at Superfund sites must satisfy
two "threshold" criteria specified in ihe NCP to be
eligible for selection: 1) the remedy must be protec-
tive of human health and the environment; and 2) the
remedy must meet (or provide ihe basis for waiving)
the ARARs identified for the action.* There generally
are several different types of ARARs associated with
ground-water remedies at Superfund sites, such as re-
quirements For discharge of treated water to surface
wafer bodies or other receptors, limitations on rein-
jection of treated water inio ihe subsurface, and
cleanup levels for contaminants in the ground water,
ARARs used to establish cleanup levels for current or
potentially drinkable ground water typically are
MCLs or non-zero MCLGs established under the
Federal Safe Drinking Water Act, or in sonic cases,
more stringent State requirements. For compounds
for which there are no ARARs, cleanup levels gener-
ally are chosen lo protect users or receptors from un-
acceptable cancer and non-cancer health risks or ad-
verse environmental effects, Such levels generally
are established to fall within the range of 1CT to 10'*
lifetime cancer risk or below a hazard index of one
for non-carcinogens, as appropriate,
ARARs may be waived by EPA for any of the six
reasons specified by CERCLA and UK NCP (High-
light I), including technical impracticability from
an engineering perspective. TI waivers generally
will be applicable only for ARARs thai are used to
establish cleanup performance standards or levels,
such as chemical-specific MCLs ot State ground-wa-
ter quality criteria.
Highlight i.
CERCLA ARAR Waivers
The six ARAR waivers provided by CERCLA
§12l(dX4)ara:
1, Interim Action Waiver;
2. Equivalent Standard of Performance Waiver;
3, Greater Risk to Health and ihe Environment
Waiver;
4. Technical Impracticability Waiver;
5, Inconsistent Application of Suite Standard
Waiver; and
6, Fund Balancing Waiver,
8 NCP §3tK.).4;}Q(0( 1 )(i). For a detailed discussion of the Superfund remedy selection process, see also EPA l9S8a and I938b.
-------
Use of (he term "engineering perspective" implies that
a TI determination should primarily focus on the tech-
nical capability of achieving the cleanup level, with
cost playing a subordinate role, The NCP Preamble
states thai Tl determinations should be based on;
"...engineering feasibility and reliability, with
cost generally not a major Factor unless compli-
ance would be inordinately costly."9
4J.2 RCRA
The Proposed Subpart S rule specifies that the correc-
tive action for contaminated ground water include at-
tainment of "nrediw cleanup standards," which gener-
ally are Federal or State MCLs, contaminant levels
within the range of 10"4 to 1Q"*J lifetime cancer risk, or
Iw.ard index of less than one for non-carcinogens, as
appropriate. The proposed rule also specifies three
conditions under which aiuiinmcni of media cleanup
standards may not be required; 1} remediation of the- re-
lease would provide no significant recluciion in risks to
actual or potential receptors; 2) the release docs not oc-
cur in, or threaten, ground waters thai are current or po-
tential sources of drinking water; and 3) remediation
of the release to media cleanup standards is tech-
nically impracticable."
Further clarification of TI determinations is provided
in ihe preamble 10 the proposed rule. The determina-
tion involves a consideration of the "engineering
feasibility and reliability" of attaining media
cleanup standards, as well as situations where reme-
diation may be "technically possible," but the "scaie
of the operations re-quired might be of such a magni-
tude and complexity ihai the alternative would be
impracticable" (emphasis added).1'-
The basis for a RCRA Subpart S Tl decision (engineer-
ing feasibility, reliability, and the magnitude and com-
plexity of the action) therefore k consistent with ihai
provided for the Superfund program in the NCP, In the
context of remedy selection, boih programs consider
the notion of technical feasibility along with reliability
and economic considerations; however, the role of cost
tor scale] of the action is subordinate to the goal of
remedy proioclivene&s,
4.2 Timing of Tl Decisions
Tl decisions may be made either when a final site
decision document is being developed (e.g., RCRA
Statement of Basis and Response to Comments or
Superfund ROD) or after the remedy has been
implemented and monitored for a period of lime,
EPA believes that, in many cases, TI decision should
be made only after interim or full-scale aquifer
remediation systems are implemented because often it
is difficult to predict the effectiveness of remedies
based on limited site characierixation data alone.
However, sit some cases, TI decisions may be made
prior to remedy implementation. These pro-
implementation or "front-end" TI decisions must be
supported adequately by detailed site characterization
and data analysis. Front-end TI evaluations should
focus on those data and analyses Lhai define the most
critical limitations to ground-water restoration.
Data and analysis requirements for front-end deci-
sions should be considered care fulls, Generally, in-
formation regarding the nature and extent of contami-
nation sources is more critical to assessing restoration
potential than are other types of characterization data.
This often is die case, as currently available technolo-
gies generally are more effective for remediating and
restoring contaminated aquifers affected only by dis-
solved, or aqueous, contamination. However, certain
type* of source contamination are resistant to extraction
by these technologies and Can continue to dissolve
slowly into ground water for indefinite periods of time,
Examples of this type of source constraint include cer-
tain occurrences of NAPl.s. such as where the quantity,
distribution, or properties of the N APL render its re-
moval from, or destruction within, ihe subsurface infea-
sibleor inordinately costly (Sec Section 3,0),
Geologic constraints, such as aquifer heterogeneity
(e.g., inlerluycring ot coarse and tine-grained strata),
also may eniically limit the ability w restore an aquifer.
However, it generally is more difficult to accurately de-
termine the impact of such constraints prior to imple-
mentation and monitoring ofparual or full-scale nqtii-
S'er remediation efforts. Some geologic constraints,
however, may be-defined suliiciciilly during site
characterization so that their impacts on restoration
potential are known wills a relatively high degree of
certainly. An example ol this type QS constraint m-
ciudes complex fracturing ul bedrock aquifers,
which makes recover.' of contaminated ground wa-
ter or DNAPLs extremely difficult.
ii should be noted, however, that die presence of
known remediation constraints, such as DNAPL.
9 Sec NCP PriiamM e. 5 5 FR 8748, March 8, 1990,
10 Technical •imprsctiuabilky is discussed in Sections 264,525{d)(2) ;md 26-1.5."51 of the I'rc
11 Proposed Suture S; 55 FR 30830, July 27, 1990,
10
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fractured bedrock, or other condition, are not by
themselves sufficient, to justify a TI determination,
Adequate site characterization data must be presented
to demonstrate, not only ihai the constraint exists, but
lhai the effect of the constraint on contaminant distri-
bution and recover)' potential poses a critical limita-
tion to the effectiveness of available technologies.
12
4.3 Tl Evaluation Components
Determinations of technical impracticability will be
made by EPA based on site-specific characterization
and, where appropriate, remedy performance data.
These data should be- collected, analyzed, and pre-
sented so that the engineering feasibility and reliabil-
ity of ground-water restoration are fully addressed in
a concise and logical manner,
The TI evaluation may be prepared by the owner/op-
erator of a RCRA facility, by § PRP at an enforce-
ment-lead Superfund site, or by EPA or the State at
Fund- or Slate-lead sites, as appropriate. The evalu-
ation generally should include the following com-
ponents, based on site-specific information and
analyses:
1. Specific ARARs or media cleanup standards for
which TI determinations are sought (See Section
4.4,1),
2, Spatial area over which the Tl decision will apply
(See Section 4,4,2).
3, Conceptual model that describes site geology, hy-
drology, ground-water contamination sources,
transport, and fate (See Section 4.4,3),
4, An evaluation of the restoration potential of the silt'.,
including data and analyses that support any
assertion ihai attainment of ARARs or media
cleanup standards is technically impracticable from
an engineering perspective* (See Section 4.4.4), At a
minimum, this generally should include:
a, A demonstration that contamination sources
have been identified and have been, or will be,
removed and contained to the extent practicable;
b. An analysis of the performance of any ongo-
ing or completed remedial actions;
c. Predictive analyses gf the umeframes to attain
required cleanup levels using available tech-
nologies; and
d. A demonstration ihai no other remedial tech-
nologies (conventional or innovative) could
reliably, logically, or feasibly attain the
cleanup levels at the site within a reasonable
dmcframe,
5. Estimates of the cost of the existing or pro-
poised remedy options, including construction,
operation, and maintenance costs (See Section
4.4,5).
6, Any additional information or analyses that
EPA deems necessary for the Tl evaluation.
The data and analyses needed to address each of
these components of a TI evaluation should be de-
termined on a site-specific bttsis. Where outside
parties are preparing die TI evaluation, its contents
generally should be identified and discussed prior to
submittal of the evaluation to EPA. Early agreement
between EPA and PRPs or owner/operators on the type
and quantity of dauiand analyses required forTI deci-
sions will promote efficient review of TI evaluations.
References to other documents in ihe administrative
record, such m the RI/FS and RF1, likely will be nec-
essary 10 produce a concise evaluation; however,
these references should be as explicit as possible
(e.g., cite specific page or table numbers). Technical
discussions and conclusions should be supported by
dam compilations, statistical analyses, or other types
of data reduction included in the evaluation.
4.4 Supporting Information tor Tl Evaluations
Most, if not all, of die information needed to evaluate
Tl could be obtained during a thorough site investiga-
tion and, where appropriate, remedy performance
monitoring efforts. Ai some sites, however, addi-
tional analysis of existing data or new information
may be required before EPA can determine accu-
rately the technical practicability oi'ihe restoration
goals. Not all of the data or analyses outlined in this
guidance will be required at all situs; specific infor-
mation needs will depend on site conditions and any
ongoing remediation efforts.
12 For this guidance a 'Tl evaSuaiion" comprises ihe date and analyses n^ussary to mike a TI determination. The Tl evaluation
may be performed by PRPs at enforcemenl-iead Sujxrfund sites, or by SUEC or other Federal agencies, where appropriate,
Similarly, owner/operators at RCRA facilities may perform TI evaluations. However, the actual Tl "determination," or "deci-
sion," will be made by EPA (or other lead agency, as appropjiate).
11
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The data and analyses identified and discussed below
address the TI evaluation components provided in
Section 4,3.
4,4.1. Specific ARARs or Media Cleanup
Standards
The TI evaluation should identify the sjKcific
ARARs or media cleanup standards (i.e., the specific
contaminants) Cor which the determination is sought
Such conutminants generally should include only
those for which attainment of the required cleanup
levels is technically impracticable. Factors EPA
wilt consider when evaluating contaminants thai
may be included in the TI decision include; 1) die
technical feasibility of restoring some of the con-
lam i nan is present in ihe ground water; and 2) the
potential advantages of attaining cleanup levels for
some of the contaminants.
For example, consider a Superfund site with a DNAPL
contamination problem (e.g., TCE), including a wide-
spread subsurface DNAPL source area for which con-
tainment or restoration are technically impracticable.
The aqueous plume also contains inorganic contamina-
tion (e.g., chromium) from on-she sources, Although it
would be feasible to reduce ctiromium concentrations
to the required elteinup level within a reasonable time-
frame, TCE concentrations would remain above
cleanup levels much longer due to ihe continued pres-
ence of the DNAPL or slow desorption of TCE from
aquifer materials. However, in such cases, EPA may
choose to limit iheTl ARAR waiver to TCE alone,
while requiring cleanup of the chromium.13
Two situations would favor use of this approach,
The first would be where attaining chromium cleanup
levels in the ground water will make future ex situ
treatment of ihe (TCE-contaminated) ground water
less complex and less expensive. This may be advan-
tageous where a community wishes to extract the
TCE-coniainmaied water, perform ex situ treatment,
and put the treated water to beneficial use, A related
consideration is whether removal of the chromium
will facilitate future subsurface remediation using a
newly developed technology. The second situation
favoring this approach is where one of the contami-
nants (e.g., TCE) is being naturally biodcgtaded and
ihe other (e.g., chromium) is not. Therefore, cleanup
of the chromium may result in more rapid attainment.
of die long-term cleanup goals at the site.
Where the balance of conditions at such a site do not
indicate that it is practicable to attain the cleanup
levels for only some of the contaminants present.
EPA may conclude thai cleanup levels for the re-
maining contaminants need not be attained, depend-
ing on the circumstances of Use silc. As discussed
luriher in Section 5,0, however, this decision does
not preclude, EPA from selecting (or continuing op-
eration of) a remedy that includes active measures
{e.g., puinp-and-trcal) along with measures to pre-
vent exposure (e.g., institutional controls) needed to
address site risks.
4.4.2 Spatial Extent 0/77 Decisions
The TI evaluation should specify the horizontal and
vertical extent of the area for which the TI determina-
tion is sought, Where EPA determines that ground-
waier restoration is technically impracticable, the
area over which the decision applies (ihe "TI zone")
generally will include all portions of the contami-
nated ground water that do not meei ihe required
cleanup levels (contaminated ground-water zone), un-
less the TI zone is otherwise defined by EPA.
In certain cases, EPA may restrict the extent of die
TI zone 10 a portion or subarea within ihe contami-
nated ground-water /otic. For example, consider a
DNAPL site where it is technically impracticable to
remove the residual DNAPLs from the subsurface
but it is feasible and practicable to: 1} limit further
migration of contaminated ground-svaier using a
containment system; and 2) restore ihat portion of
ihe aqueous plume outside of the containment area,
The TI /.one in this case should be. restricted to that
portion of the site that lies within die containment
area. Outside ol the TI /.one, ARARs or media
cleanup standards still would apply. The potential
to spatially restrict the T! zone, therefore, will de-
pend on the ability to delineate and contain non-re-
movable subsurface contamination sources and re-
store those portions ol" die aqueous plume outside of
Use conuiinmeni area, The spatial extent of the TI
zone should be limited to as small an area as pos-
sible, given the circumstances of the site.
A TI /.one should be delineated spatially, bolh in area
and depth, Depth of a TI zone may be defined in ab-
solute terms (e.g., feet above mean sea level) or in
relative terms (e.g., with respect to various aquifers
within mulii 'aquifer systems), as appropriate. Where
13 The extracted pound water would ltke»y need to be treated fur htxh TCT. and chromium to satisfy treatment ana waste db;-
tMsal ARARs.
12
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the TI /.one will be restricted u> a portion of the con-
laminaicd ground-water zone, ihc limits of Uie Tl
zone should be- delineated clearly on site maps and
geologic cross-sections. Delineation of the TI zone
based on the location of a particular mapped contami-
nant concentration contour interval (e.g., the 200 part
per billion isoconcentration line) generally should be
avoided. This is because the location of such mapped
contours often is highly interpretive, arid their posi-
tion may change with time. While concentration data
may be appropriate to consider when determining the
sixe of a containment area or the extent of a TI zone,
the limits of thai Tl zone should be fixed in space,
both horizontally and vertically.
4,4.3 Development and Purpose of (he Site
Conceptual Model
Decisions regarding the technical practicability of
ground-water restoration must be based on a thor-
ough characterization of the physical and chemical
aspects of the site. Characterization data should de-
scribe site geology and hydrology; contamination
sources, properties, and distribution; release mecha-
nisms and rates; fate and transport processes; current
or potential receptors; and oiher elements that define
the contamination problem and facilitate analysis of
site restoration potential. While the elements of such
a mo-del may vary from site to site, some generaliza-
tions can be made about what such a model would
contain. Examples of these elements are provided in
Figure 4. The she conceptual model synthesizes data
acquired from historical research, site characteriza-
tion, and remediation system operation.
The site conceptual model typically is presented as a
summary or specific component of a site investigation
report. The mode! is based on, and should be sup-
ported by, interpretive graphics, reduced and analyzed
data, subsurface investigation togs, and oilier pertinent
characteri/aiion information. The site conceptual
mode! is not a mathematical or computer model, al-
though these may bu used to assist in developing and
testing the validity of a conceptual model or evaluating
the restoration potential of ihe site. The conceptual
model, like any theory or hypothesis, is a dynamic tool
dial should be tested and refined throughout the life of
the project. As illustrated in Figure 5, the model should
evolve in stages as information is gathered during the
various phases of site remediation, This iterative pro-
cess allows data collection efforts to be designed so
that key model hypodieses may be tested and revised to
reflect new information.
The conceptual model serves as the foundation for
evaluating the restoration potential of the sile and,
thereby, technical impracticability as well. The TI
determination must consider how site conditions im-
pact the- potential for achieving remediation goals and
whether remediation performance, cost-effectiveness,
and timcframe meet EPA requirements or expecta-
tions. As these determinations rely on professional
judgment, the clarity of ihe conceptual model (and
supporting information) is critical to the decision-
making process.
4.4,4 Evaluation of Restoration Potential
4.4.4,1 Source Control Measures. Remediation of
contamination sources is critical to the success of
aquifer restoration efforts. Continued releases of
contamination from source materials to ground water
Can greatly reduce the effectiveness of aquifer resto-
ration technologies, such as pmnp-aruJ-ueat, which
generally are effective only for removing dissolved
contaminants (EPA I989b; I992d). EPA considers
subsurface NAPLs to be source materials because
they are capable of releasing significant quantities of
dissolved contamination to ground water over long
periods of time.
A demonstration that ground-water restoration is
technically impracticable generally should be- accom-
panied by a demonstration thai contamination sources
have been, or will be, identified and removed or
treated to the extent practicable. EPA recognizes that
locating and remediating subsurface sources can be
difficult, Fur example, locating DN APLs in certain
complex geologic environments may be impracti-
cable, EPA expects, however, that all reasonable ef-
forts will be made to identify the location us" source
areas through historical information searches and site
c harac teri/al ion efforts,
Source removal ynd remediation may be difficult,
even where source locations are known. The appro-
priate level of effort for source removal and remedia-
tion musi be evaluated on a site-specific basis, con-
sidering the degree of risk reduction and any oihcr
potential benefits that would result from such an ac-
tion, Even partial removal of contamination sources
can greatly reduce ihi- long-term reliance on both ac-
tive and passive ground-water remediation,
Where complete source removal or treatment is im-
practicable, use of migration control or containment
measures should be considered. Physical and hy-
draulic barriers are proven technologies that are ca-
pable of limiting or preventing further contaminant
13
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Figure 4, Elements of Site Conceptual Model
The data and analysis required for TI evaluations will be determined by EPA on a site-specific basis. This infor-
maiion should be presented in formats conducive to analysis and in sufficient detail to define the key siic condi-
tions and mechanisms in at limit restoration potential. Types of information and analysis that may be needed for
conceptual model development are illustrated below.
Background Information
Location of water supply wells.
Ground-water Classification,
Nearby wellhead protection areas or sale-source
Location of potential environmental receptors.
Geologic and Hydrologlc Information
Description of regional and sits geology.
Physical properties of subsurface materials
(e.g., texture, porosisy, bulk density).
Stratigraphy, including thickness, lateral extent, contin-
uity of units, and presence of depcsitional features,
such as channel deposits, thai may provide preferential
pathways for, or barriers to, contaminant transport.
Geologic structures that may form preferential pathways
tor NAPL migration or zones o! accumulation.
Depth 10 ground water.
Hydraulic gradients (horizontal and vertical).
Hydraulic properties of subsurface materials (e.g.,
hydraulic conductivity, storage coefficient, effective
porosity) and their directional variability (anisotropy).
Spatial distribution of sol or bedrock physical/hydraulic
properties (degree of heterogeneity).
Characterization of secondary porosity features
(e.g., fractures, karst features) to the extent practicable,
Temporal variability in hydrologtc conditions,
Ground-water recharge and discharge information.
Ground-water/surface water interactions.
Contaminant Source and Release Information
• Location, nature, and history of previous
contaminant ralsases or sources.
* Locations and characterizations of continuing
releases or sources,
• Locations of subsurface sources (e.g., NAPLsj,
Contaminant Distribution, Trinsport, ind Fate Parameters
Phase distribution of each contaminant (gaseous, aqueous, sorbed, free-phase NAPL, or residua! NAPL)
in ihe unsalurated and saturated zones,
Spatial distribution of subsurface contaminants in each phase in the unsaturafed and saturated zones.
Estimates of subsurface 'Contaminant mass.
Temporal '.rends in contaminant concentrations in eacn phase,
Sorption information, including contaminant retardation factors.
Contaminant transformation processes and rate estimates,
Contaminant migration fates.
Assessment of facilitated transport mechanisms (e.g., colloidal transport),
Properties of NAPLs that affect iransport (e.g., composition, effective constituent solubilities, density, viscosity).
Geochemica) characteristics o! subsurface media that affect contaminant transport and fate.
Other characteristics that affect distribution, transport, and fa?a (e.g., vapor transport properties).
14
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Figure S, Evolution of the Site Conceptual Model
Site Background and History
Preliminary Site Investigations
Likely Sources and
'Receptors Identified'
Conceptual Model
Provides Basis for:
Early Action/Removal of
Near-Surface Materials
Site Characterization Studies
(RI/FS, RF1)
Removal of Subsurface Sources
(e.g., free-phase NAPLs)
Conceptual Model
Provides Basis for:
Pilot Studies
Interim Ground-Water Actions
1
Conceptual Model
Provides Basis for;
Evaluation of Restoration Potential
(or 11)
Full-Scale Treatment System
Design and Implementation
Performance Jvfonitoring and
Evaluations
Enhancement or Augmentation of
Remediation System, if Required
Future Evaluation of TI, if
Required (See Figure 6)
Excavation and
Drum and Soil CappingofLagoon
Removak
Mon
Installation of
Subsurface
toring Systems
Pilot
Studie;
Interim Action
Hydraulic
Containment
15
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migration from a source area under the right circum-
stances. While these containment measures sire not
capable of restoring source areas LO required cleanup
levels (i.e., a TI decision may be necessary for the
source area), they may enable restoration of portions
of (he aquifer ouissde the containment zone.
4,4.4.2 Remedial Action Performance Analysis,
The suitability ant! performance of any completed or
ongoing ground-water remedial actions should be
evaluated with respect U) the objectives of those ac-
tions. Examples of remedy performance data are pro-
vided in Figure 6. The performance analysis should:
1, Demonstrate thai the ground-water monitoring pro-
gram within and outside of she aqueous contaminant
plume is of sufficient quality and detail to fully
evaluate remedial action performance (e.g., to ana-
lyze plume migration or containment and identify
concentration trends within the remediation zone}.'4
2. Demonstrate lhai the existing remedy has been ef-
fectively operated and adequately maintained.
3. Describe and evaluate die effectiveness of any
remedy modifications (whether variations in op-
eration, physical changes, or augmentations lo the
system) designed lo enhance its performance,
4. Evaluate trends in subsurface contaminant concen-
trations. Consider such factors as whether the aque-
ous plume has been contained, whether the areal ex-
tent of the plume is being reduced, and the rates of
contaminant concentration decline ami contaminant
mass removal. Further considerations include
whether aqueous-phase eonccnuntions rebound
when die system is shut down, whether dilution or
other natural attenuation processes are responsible
for observed trends, and whether contaminated soils
on site are contaminating the ground water.
Analysis of aqueous-phase concentration dam should
be performed with caution. Contaminant concentra-
tions plotted as a function of lime, pore volumes of
flushed fluids, or other appropriate variables may be
useful in evaluating dominant contaminant fate and
transport processes, evaluating remedial system design,
and predicting future remedial system performance,
Sampling methodologies, locauons, arid strategics,
however, should be analyzed to determine the impact
they may have had on observed concentration trends.
For example, studies of ground-water extraction sys-
tems indicate thai some systems show rapid initial
decreases in aquifer concentration, followed by less
dramatic decreases lhat eventually approach an as-
ymptotic ajm:eniraiion level (EPA 19S9b, I992d),
This "leveling off effect may represent either a
physical limitation to further remediation (e.g., con-
taminant diffusion from low permeability units) or an
artifact of the system design or monitoring program.
Professional judgment must be applied carefully
when drawing conclusions concerning restoration po-
tential from this information.
In certain cases, EPA may determine lhat lack of
progress in achieving the required cleanup levels has
resulted from system design inadequacies, poor sys-
tem operation, or unsuitability of the technology for
site conditions. Such system-related constraints are
not sufficient grounds for determining that ground-
water restoration is technically impracticable. In
such instances, EPA generally will require diat the
existing remedy be enhanced, augmented, or replaced
by a different technology. Furthermore, EPA may re-
quire modification or replacement uf an existing rem-
edy 10 ensure protectiveness, regardless of whether or
not attainment of required cleanup levels is techni-
cally impracticable.
4.4,4,3 Restoration Timefraine Analysis. Estimates
of the time frame required lo achieve ground-water
restoration may be considered in TI evaluations.
While restoration timeframes may be an important
consideration in remedy selection, no single
timeframe can be specified during which restoration
must be achieved to be considered technically practi-
cable.. However, very long restoration timeframes
(e.g., longer than 100 years) may be indicative of
hydrogeologie or conuiminant-related constraints lo
remediation. While predictions of restoration
timcframes may be useful in illustrating the effects of
such constraints, EPA will base TI decisions on an
overall demonstration of the extent of such physical
constraints at a site, not on restoration timeframe
analyses alone. Such demonstrations should be based
on detailed and accurate site conceptual models that
also can provide the bases for meaningful predictions
of restoration Umeframes,
14 Further guidance on design of poit'wrmance monitoring for remedial actions tt ground-water si if* is provided in "CknersiS
Methods fur Remedial Operations Perftmnisiice Evaluations," EPA Office of Research and Duvdoptnau Publication EPA,'
6GO/R-92/OOZ, huiuaiy 1992 ;EPA 1992cs.
16
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Figure 6, Remedy Performance Analysis
Remedy design and performance data requirements should be specific 10 technologies employed and sue conditions.
The categories of required information normally necessary to evaluate performance arc provided below with some
examples of specific data elements. These data should be reported to EPA in formats conducive to analysis and in-
terpretation. Simple data compilations are insufficient for [his purpose.
Remedy Design and Operational information
Design and as-buil! construction information,
including locations ot extraction or in sftv treat*
merit points with respect to {he contamination.
Supporting design calculations (e.g., calculation of
wall spacing}.
Operating information pertinent to remedy [e.g.,
records of the quantity and quality of extracted or
injected fluids),
Percent downtime and other maintenance
problems.
Enhancements to Original Remedial Design
Information concerning operational modifications,
such as variations in pumping, injection rates, or
locations,
Rationale, design, and as-built construction
information for aysiem enhancements.
Monitoring data and analyses that illustrate the
effect these niodilicatfQns have had on system
performance.
Ground-water
Extraction/Injection
and Performance
Monitoring Systems
Hydraulic
Containment and
Performance
Monitoring Systems
DNAPL
Recovery
System
Source Removal or Control
Source removal information (e.g., results of soil
excavations, removal of lagoon sediments, NAPL
removal activities),
Sowcs control information {e.g., results of NAPL
containment, capping of former waste manage-
ment units).
Performance Monitoring Information
Design and as-built construction intormation for
performance monitoring systems.
Hydraulic gradients and other information
demonstrating piuma containment or changes in
areal extent or volume,
Trends in subsurface contaminant concentrations
determined at several/many appropriate locations
in the subsurface. Trends should be displayed as
a function of lime, a function of pora volumes ol
flushed fluids, or older appropriate measures.
Information on types and quantities of
contaminant mass removed and removal rates,
17
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A furilier consideration regarding the usefulness of
restoration uniefranie predictions in TI evaluations is
the uncertainty inherent in such analyses. Restora-
tion timefraraes generally are estimated using math-
ematical models that simulate the behavior of subsur-
face hydroiogic processes, Models range from those
with relatively limited input data requirements that
perform basic simulations of ground-water flow only,
to those with extensive data requirements thai are. ca-
pable of simulating multi-phase flow (e,g,, water,
NAFL, vapor) or other processes such as contaminant
adsorption 10, and desorption from, aquifer materials.
Model input parameters generally are a tombination
of values measured during site characterization stud-
ies and values assumed based on scientific literature
or professional judgment. The input parameter selec-
tion process, a> well as the simplifying assumptions
ol'lhc mathematical model itself, result m uncertainty
of the accuracy of the output, Restoration tinneframes
predicted using even the rnosi sophisticaied modeling
tools and data, therefore, will have some degree of
uncertainly associated wiih them.
Restoration timcframe analyses, therefore, generally
are well suited for comparing two or more remedia-
tion design alternatives to determine the most appro-
priate strategy for a particular site. Where cm-
ployed for such purposes, restoration limeframe
analyses should be accompanied by a thorough dis-
cussion of all assumptions, including a list of mea-
sured or assumed parameters and a quantitative
analysis, where appropriate, of the degree of uncer-
tainly in those parameters and in the resulting time-
frame predictions. The uncertainty in the predic-
tions should be factored into the weight they are
given in the remedy decision process.
4,4.4.4 Other Applicable Technologies. The Tl
evaluation should include a demonstration that no
Oilier remedial technologies or strategies would be
capable of achieving ground-water restoration at the
site,15 The type of demonstration required will de-
pend on the circumstances of the site and the state, of
ground -water remediation science at the lirne such an
evaluation is made. In general, EPA expects thai
Such a demonstration should consist of; 1) a review
of the technical literature to identify candidate tech-
nologies; 2} a screening of the candidate technologies
based on genera! site conditions to identify poten-
tially applicable technologies; and 3) an analysis, us-
ing site hydrogeologie and chemical data, of the ca-
pability of any of she applicable technologies to
achieve ihe required cleanup standards. Analysis of
the potentially applicable technologies generally tan
be performed as a "paper study," EPA, however, may
reserve the right 10 require tieaiability or pilot testing
demonstrations to determine ihc actual effectiveness
of a technology at a pariivulur site,
Treaiabiltty and pilot testing should be conducted
witli rigorous controls and mass balance constraints,
Information required by P. PA for evaluation of pilot
tests will be similar to that required tor evaluation of
existing remedial] on systems (e.g., detailed design
and performance data),
4.4.4.5 Additional Considerations. Techniques
used for evaluation of ground-water restoration
potential are still evolving. The results of such
evaluations generally will have some level of
uncertainty associated with them. Interpretation of
the results of restoration potential evaluations,
therefore, will require die use of professional
judgment The use of mathematical models and
calculations of masi removal rates are two cxaniplei uf
techniques Lliat require particular caution.
Oiu,.iupJ " tUtr I li- v viul C jiiui'i'irKint Tr^r.^yt.Ftile
-Ln •, ji predating d^sgn r* nor-
t .. it 1 [i lU Hi»"is »> K i a^ ti nLa'iu-
nart ^J' e I • jn n a uiti iv iru nvd-auli. L jn-
dutnr, I ^J .b u v Mj^h L.- ne ti jghitudt Ljid J. -
LnbuU mil ^ i' MI i !,„ t nu HIT 'nu , r i.i.ritr,itnini,
thcretoic. v>-nl mvuive unccitainty. Ine souice ana
degree, of this uncertainty should be described, quanti-
fied. and evalualL'd wherever passible sy the reviewer
understands the level of confidence that should be
placed in the predicted concentration values or other
outputs. Predictive modeling Jfiay be most valualsle in
providing insight into processes that dominate conuuru-
nant transport and fate at the site and evaluating the
relative i'ffcctivene% of different remedial alientatives.
Further guidance and information on the use of
ground-water models is provided in Anderson and
Woessner (1992), HPA (I992f), and EPA (I992g),
Cojitaminant_Majs Removal Hsumates. Evaluation of
coniammani mais removal mav be useful at some siics
15 See
jw in the NCP (55 £E &^», March S. 1990) and Subpart S ;55 FR 3ui3S. July 27,
18
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with existing remediation systems. "These measures
may include evaluation of mass removal rates,
comparison of removal rates to in situ mass esti-
mates, changes in the size of the contaminated area,
comparison of mass removal rates wiih pumping rates,
and comparison of such measures with associated
costs, Mass removal and balance estimates should be
used with caution, as there often is a high degree of
uncertainty associated with estimates of the initial mass
released and the mass remaining in situ, This uncer-
tainty results from inaccuracy of historical site waste-
management records, subsurface, heterogeneities, and
the difficulty in delineating the severity and extent of
Subsurface contamination,
4,4,5 Cost Estimate
Estimates of the cost of remedy alternatives should
be provided in the TI evaluation. The estimates
should include the present worth of construction, op-
eration, and maintenance costs. Estimates should be
provided for the continued operation of the existing
remedy (if the evaluation is conducted following
implementation of ihc remedy) or for any proposed
alternative remedial strategies,
As discussed in Section 4,4,1, a Superfund remedy
alternative may be determined to be technically im-
practicable if lite cost of attaining ARARs would be
inordinately high. The role of cost, however, is sub-
ordinate to that of ensuring proieeiiveness. The point
at which the cost of ARAR compliance becomes in-
ordinate must be determined based on the particular
circumstances of the site, As with long restoration
tirneframes, relatively high restoration costs may be
appropriate in certain cases, depending on the nature
of the contamination, problem and considerations
such as the current and likely future use of the ground
water, Compliance with ARARs is not subject U> a
cost-benefit analysis, however."'
5,0 Alternative Remedial Strategies
5,1 Options and Objectives for Alternative
Strategies17
EPA's goal of restoring contaminated ground water
within a reasonable timeframe at Superfund or RCRA
16 A Fund-Balancing ARAR waiver may be invoked at Fund-lead Superfund sites where meeting an ARAR would entail such
cost in relation to the added degree of protcciion <;r reduction of risk that remedial actions as other sites would be jeopardized
(EPA !9S9c),
17 These reeommendaikms are consistent with those made in Section 31) concerning PNAPL sites, but fire applicable lor any
ske where reiloraison is technically imprac-iciible.
18 PRPs or owncr/upwators may propose and analyze alternative remedial sirawgies. However, only EPA {or designated fcad
agency, where appropriate) has remedy sdmiw: auihoriiy.
sites will be modified where complete restoration is
found to be technically impracticable. In such cases,
EPA will select an alternative remedial strategy that
is technically practicable, protective of human health
and the environment, and satisfies the statutory and
regulatory requirements of the Superfund or RCRA
programs, as appropriate/8
Where a TI decision is made ai the "front end" of the
site remediation process (before a final remedy has
been identified and implemented), the alternative
strategy should be incorporated into a final remedy
decision document, such as a Superfund ROD or
RCRA permit or enforcement order. Where the TI
decision is made after ihe Tina] decision document
has been signed (i.e., after a remedy has been imple-
mented and its performance evaluated), the alterna-
tive remedial strategy should be incorporated in a
modified final remedy decision document, such as a
ROD amendment or RCRA permit/order modifica-
tion (see Section 6,0),
Alternative remedial strategies typically will address
three types of problems at contaminated ground-wa-
ter sites: prevention of exposure to contaminated
ground water; remediation of contamination sources;
and remediation of aqueous contaminant plumes,
Recommended objectives and options for addressing
these three problems are discussed below. Note that
combinations of two or more options may be appro-
priate at any given site, depending on the size and
complexity of the contamination problem or oilier
site circumstances.
S.I.i Exposure Control
Since the primary objective of any remedial strategy
is overall proteciiveness, exposure prevention may
play a significant role in an alternative remedial strat-
egy. Exposure control may be provided using institu-
tional Controls, such as deed notifications and restric-
tions on water-supply well construction and use. The
remedy should provide assurance that these measures
are enforceable and consistent with State or local
laws and ordinances.
5.1.2 Source Control
Source, remediation and control should be considered
when developing an alternative remedial strategy.
19
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Sources should be located and treated or removed
whew feasible-and where significant risk reduction will
result, regardless of whether EPA lias determined that
ground-water restoration is technically impracticable.
In some cases, however, the inability to remove or
treat sources will be a major factor in a Tl decision,
Where sources cannot be completely treated or re-
moved, effective source containment may be- critical
'o the long-term effectiveness and reliability of an al-
ternative ground-water remedy, Options currently
available for source containment usually involve ei-
ther a physical barrier system (such as a slurry wall)
or a hydraulic Containment system (typically a pump-
and-u-an system) (EPA t992b),
Applicability arid effectiveness of containment sys-
lem.s arc influenced by several hydrogeologic i'aciors,
however. For example, the effectiveness of a slurry
wail generally depends on whether a continuous. low
permeability layer exists at a relatively shallow depth
beneath the site.
Source- fontainme.nl has several benefits. First,
source- containment will contribute to ihe long-term
management of contaminant migration by limiting
the further contamination of ground water and spread
of potentially mobile sources, such as NAPLs. Sec-
ond, effective source containment may permit resto-
ration of that portion of the aqueous plume that lies
outside of the containment area. Third, effective
containment may facilitate the future use of new
source removal technologies, as some of these tech-
nologies (e.g., surfactants, steam injection, radio fre-
quency heating) may increase the mobility of residual
and iree-pfmse NAPLs, Remobilizauon of NAPLs,
particularly DNAPLs, often presents a significant risk
unless the source area can be reliably contained.
5,/J Aqueous Plume Remediation
Remediation of the aqueous plume is the third major
technical concern of an alternative remedial strategy.
Where the technical constraints to restoration include
the inability to remove contamination sources, the
ability 10 effectively contain those sources will be
critical lo establishing the objectives of plume
remediation. Where- sources can be effectively con-
tained, the portion of the aqueous plume outside of
the containment area generally should be restored to
the required cleanup levels.
Inability to contain the sources, or oilier technical
constraints, may render plume, restoration technically
impracticable. There arc several options for altenm-
live remedial strategies in such cases. These include
hydraulic containment of the leading edge of the
aqueous plume, establishing a less-siringcm cleanup
level diat would be actively sought throughout the
plume (at Superfund shes), and natural attenuation or
natural gradient flushing of the plums.
Containment of the aqueous plume usually requires
the pumping and treating of contaminated ground wa-
ter, but usually involves fewer wells and smaller
quantities of water than does a full plume restoration
effort. Plume containment offers the potential advan-
tages of preventing further spreading of the contami-
nated ground water, thereby limiting the size of the
plume, and preventing the plume from encroaching
Oft water-supply wells or discharging to ecologically
sensitive areas,
At certain Superfund sites, it may be feasible to re-
store (he- contaminated plume (outside ol any source
containment area) to a site-specific cleanup level that
is less stringent than that originally identified, EPA
may establish such a level as the cleanup level within
the Tl zone, where appropriate. The site-specific.
level may consider the targeted risk level for site
cleanup and other factors. Site -specific cleanup lev-
els offer the advantage of providing a clear goal
against which to measure the progress of the alterna-
tive remedial strategy. However, where siie-ipeeifie
cleanup levels exceed the acceptable risk range for
human or environmental exposure, the remedy gener-
ally must include other measures {e.g., institutional
controls) to ensure proteciiveness,
At some Superfund sites, a less-stnngeru ARAk
the one determined to be unattainable may have to be
complied with, for example, it may be technically
impracticable to attain the most stringent AR/\R at a
site (e.g., a State requirement to restore ground wuier
to background concentration levels). However, the
next mosl stringent ARAR (e.g., Federal MCL) for die
same compound may be attainable. In such eases, the
next most stringent ARAR generally must be attained.
In certain situations where restoration is technically
impracticable, EPA may choose natural attenuaiton
as a component of die remedy liir the aqueous
plume/9 Natural attenuation generally will result in
Tedinieal impracticability of nis'oraiion is not a precondition to? die use of natural altcnusiion in - ^round-w^icr renwiv. ho
20
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attainment of the desired cleanup levels, but may take
longer to meei them than active remediation. This
approach is most likely to be appropriate where the
affected ground water is not a current or reasonably
expected future source of drinking water, and ground-
water discharge does not significantly impact surface
water or ecologic resources. Sufficient technical in-
formation and supporting data mosi be presented to
demonstrate the effectiveness of this strategy, along
with assurances that any institutional controls re-
quired to prevent exposure will be reliable and en-
forceable. Contingencies for additional or more ac-
tive remediation also should be incorporated into the
remedy, to be triggered by specific contaminant con-
centration levels in die site ground-water monitoring
network, or other criteria as appropriate.
5.2 Alternative Remedy Selection
The .alternative remedial strategy options discussed
above represent a range of resjxjnses for addressing the
various aspects of a ground-water contamination site.
Selection of the up lions appropriate for a particular site
must not only consider ihe desired remediation objec-
tives, as discussed above, but also the statutory and
regulatory requirements applicable to the program un-
der which the action is being taken. These require-
ments are discussed briefly below, Further information
aiKl guidance on these requirements can be obtained
from publications referenced in (his section.
52.1, Superfund
The. selection of an alternative remedy at a Superfund
site should follow the remedy selection process pro-
vided in NCP $300.430(0. Regardless of whether
ARARs are waived at the sue, the alternative remedy
slili must satisfy me two threshold remedy selection
criteria (protect human health and the environment
and comply with all ARARs that have not been
waived): be cost effective; and utilize permanent so-
lutions and treatment to ihe maximum extent practi-
cable. This last finding is saiisfied by identifying the
alternative that best balances the trade-offs with re-
spect to the remaining balancing and modifying crite-
ria, taking into account the demonstrated technics!
limitations (sec Highlight 2),20
Where ground-water ARARs are waived at a Supef-
fund site due to technical impracticability, EPA's
general expectations are to prevent further migration
of the contaminated ground-water plume, prevent ex-
posure to ihe contaminated ground water, and evalu-
ate further risk reduction measures as appropriate.
(NCP S30Q,43Ci(a)(l}(iii)(F)). These expectations
should be evaluated along with the nine remedy se-
lection criteria to determine the most appropriate re-
medial strategy for the site-
Highlight 2.
Superfund Remedy Selection Criteria
Threshold Criteria
• Overall protection of human health and
the environment
• Compliance with (or justification for a waiver
of) ARARs
Balancing Criteria
• Long-term effectiveness and permanence
• Reduction of mobility, to* icily, or volume
• Shon-ienn effectiveness
• Implemeniability
• Cost
Modifying Criteria
• State acceptance
« Community acceptance
5.2.2 KCKA
At RCRA facilities where ground-water restoration is
technically impracticable, ihe permit or order sched-
ule of compliance may be modified by establishing:
1} further measures that may be required of the per-
mittee to control exposure to residual contamination,
as necessary to protect human heal ill and ihe environ-
ment: and 2) alternate levels or measures for cleaning
up contaminated media.21
Criteria for establishing an alternative remedial strat-
egy under RCRA arc presented in Highlight 3. In ad-
dition to satisfying, ihe general standards for rem-
edies, the altemalive remedial strategy at a RCRA fa-
cility also should provide die test balance of irade-olis
among she five remedy selection decision factors,13
20 For furthdi guidance on ihe Superftmd remedy selection process, sec NCP §30G..130{f) and "Guidance for Com!ycling Reme-
dial Investigations and Feasibility Studies under CERCLA," (EPA I988a).
2; Proposed Sub-part S Rule, §264,531 fb).
22 Further guidance on remedy selection ac RCRA facilities is provided in the proposed Sub-pan S Ru!e (55 pft 30S23-3tiP>24,
July 27, 1990}.
21
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Highlight 3,
RCRA Remedy Standards and
Selection Factors
General Standards for Remedies
1 , Overall protection of human health and the
environment
2. Attainment of media cleanup standards
3. Source control
4. Compliance with waste management standards
Remedy Selection Decision
1. Long-term effectiveness
2, Reduction of waste toxicity , mobility, or volume
3, Short-term effectiveness
4. Impkmcntabiliiy
5. Cost
52.3 Additional Remedy Selection
Considerations
The choice among available remedial strategy options
may involve a consideration of the aggressiveness of
the remedy, a concept that includes both the choice of
remedial technologies as well as the relative intensity
of how that technology is applied at the site. For ex-
ample, consider a site where source area restoration is
technically impracticable but source containment is
both feasible and practicable. With the contaminant
source contained, restoration of the portion of the
plume outside of the containment area may be fea-
sible. However, as discussed earlier, there are several
options for attaining cleanup levels within the aque-
ous plume: active piimp-and-imu throughout the
aqueous plume; natural gradient flushing of the
plume towards a pump-and-trcat capture system lo-
cated at the leading edge of the plume; and natural at-
tenuation (dilution, dispersion, and any natural degra-
dation processes active within the affected aquifer).
Each alternative will attain the required cleanup lev-
els, but the choice involves a trade-off among several
factors, including: I) remediation timeframe (longer
with less aggressive strategies); 2} cost (lower with less
aggressive strategies); and 3) potential risk of exposure
(may increase with less aggressive strategies)."0
Conditions favoring more aggressive strategies (i.e.,
active pump-and-treat throughout the aqueous plume)
include the following:
1) The aggressive strategy clearly will result in a
significantly shorter restoration timcfrume than
other available options. Tnis will depend on site
hydrogeologic and contaminant-related factors, in-
cluding the complexity of the aquifer system, natural
rate of ground-water flow, quantity of surbed con-
taminant ma.ss in thy aquifer (and its rate ofdesorp-
tion), and oilier factors,
2} A shorter remediation timer'nime is desired to
reduce- the potential for human exposure, This
generally is the case where there is current or reason-
ably expected near-lent) future use of the ground wa-
ter. Factors thai may be useful in evaluating the like-
lihood of exposure include the State (or Federal, as
appropriate) classification of the ground water; avail-
ability of alternate supplies, such as municipal hook-
ups or other water supply aquifers; interconnections
of the contaminated aquifer with other surface or
ground waters; and the ability of institutional conlrols
to limit exposure.
.5,1 A shorter remediation tirneframe is desired 10
reduce ongoing or potential impacts to environ-
mental receptors. Such impacts may be caused by
discharges u> surface waters, sensitive ocologie areas
(e.g., wetlands), or sole-source aquifers,
EPA will evaluate and determine the objectives and
relative aggressiveness of ihe alternative remedy on a
site-specific basis, based on the applicable regulatory
requirements and considering the factors discussed
throughout this s-etJiyn. Where conditions favoring
more aggressive strategies do not exist, EPA is more
likely to choose a less aggressive strategy to achieve
the desired remediation, objectives. F.PA recognizes
thai, ai some sites, remedies may need to be in opera-
lion tor very long time periods. Adequate monitoring
and periodic evaluation of remedy performance
should be conducted to ensure protectiveness and lo
evaluate the nved for remedy enhancements or the
use of :ie\v or different remediation technologies.
5.2.4 Relation in Alternate Concentration
Limits
Site-specific cleanup levels established as pail of an al-
ternative remedial strategy at a Superfund site shook!
not be confused with CERCLA Alternate Concentra-
tion Limits (ACL:-;}. To qualify for use of a CERCLA
ACL, the site must meet the following three require-
ments: 1) there are known points of entry of die con-
iLimimiied ground water inu> surface water; 2) thcre
23 The Song-imn reliability of a remedy also i& an important considuiaiiuii :u: .uiemai.ve remedial stai«gy wslcciion. In this ex-
ample, long-ierax feliabtlity is primarily a function of the design and integrity of (he sovruc ouriuuimenl system.
22
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will be no statistically significant increases of the
contaminant concentrations in the surface water or
contaminant accumulations in downstream sedi-
ments; and 3) enforceable measures can be- pet into
place to prevent exposure to the contaminated ground
water (see CF.RCLA §l21(d)('2){B}(ii)), In addition,
EPA generally considers ACLs appropriate only
where cleanup to ARARs is impracticable, based on
aa analysis using the Superi'und remedy selection
"balancing" and "modifying" criteria shown in High-
light 1, Where an ACL is established, an ARAR
waiver is not necessary, Conversely, where an
ARAR is waived due to technical impracticability,
there is no need to establish a CERCl.A ACL. For
further guidance on CERCLA ACLs, refer io the
NCP Preamble (55 EE 8754, March 1990),
Site-specific cleanup levels established in response to
a Tl determination at a RCRA facility also should nol
be confused willi ACLs established as part of the
ground-water monitoring program for regulated units
under 40 CFR 264,94, ACLs established under
§264.94(;s}(3) represent concentrations that EPA de-
termines will not pose a substantial hazard to human
or environmental recepUMS. (U She ACL is exceeded,
then corrective action responsibilities for the regulated
unit itrc triggered,) A Tl determination generally will
not satisfy ihe criteria For an ACL under this authority,
6.0 Administrative Issues
6.1 II Review and Decision Process
A Tl decision must be incorporated into a site deci-
sion document (Superi'und ROD or RCRA permit or
enforcement order) or be incorporated into a modifi-
cation or amendment to an original document. In-
formation and analyses supporting the Tl decision
must be incorporated into the site administrative
record, cutter us pan of a Feasibility Study or Cor-
rective Measures Study {/or a "front-end" Tl determi-
nation) or remedy performance evaluation or other
technical report or evaluation (For a post-remedy imple-
mentation determination).
The fust step in EPA's review process for aTl determi-
nation will be u> assess the completeness and adequacy
of the Tl evaluation. Tl evaluations that do not ad-
equately address die considerations identified in this
guidance likely will have to be revised or augmented io
addrc&s ihe inadequacies identified by EPA or the re-
sponsible agency, Early consultation with EPA by
PRPs or owner/operators is encouraged to help identify
appropriate data arid analysis for the evaluation. While
a Tl evaluation is underway, remediation efforts under-
way at a site shall continue until the Suite or Federal
official responsible for the decision determines that the
existing remedy should be altered. Requirements spe-
cific io the Superi'und und RCRA programs are dis-
cussed further below,
6,J.I Super/and
As discussed in Section 4.2, Tl decisions may be
made either in Uie ROD (front-cad decisions} or after
the remedy has been implemented and monitored
(poM-implemenuttion decisions), depending on the
eux'unisuuice.sof the site,
Tl decisions at Superfund sues generally will be
made by the EPA Regional Administrator who, upon
review of a Tl evaluation, will determine whether
ground-water restoration is technically impracticable
and will identity further remedial actions io be taken
at the site, Tl determinations at Superi'und sites may
requite consultation with headquarters program man-
agement. Regional personnel should refcr Jo the
most recent OERR Remedy Delegation Memoran-
dum for current consultation requirements.24
Where a Superfund ROD will invoke a Tl ARAR
waiver ^front-end decision), EPA {01 the lead
agency) must provide notice of Us intent io waive the
.ARAR in the Proposed Plan for the sue and respond
io any State (or Federal) agency or public comments
concerning the waiver. The requirements for Slate
and community involvement tire provided in NCP
§300,500-515 and §300.430, respectively. In gen-
eral, Slate and community involvement in liie deci-
sion to waive an ARAR based on technical impracti-
cability will be the same m for oiher site remedy de-
cisions. Since Tl decisions may affect the potential
future uses of ground water, interest in Tl ARAR
waivers may be high. Therefore, it is EPA's intent to
coordinate and consult with Slates and the public re-
garding Tl ARAR waiver issues as early as possible
in the remedy decision process.
24 The lyp-e* of Superfund she remedy decisions Uiai require consultation with headquarters program management arc identified
in the" penodjealiy updated OERR Remedy Delegation Memorandum, The moss r*P--nt version available at iJic time of publi-
cation of 'Jiis guidance *as ihe "Twenty Fourth Remedy Delegation Report - FY 1993," dated February IS, 1993.
23
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State concurrence should be sought, but is not re-
quired, for all remedy decisions in which EPA in-
vokes an ARAR waiver. Where the A.RAR 10 be
waived is a Suite ARAR, EPA must notify the Suite
of this when submitting ihe RI/FS to the State or
when responding to a State-lead RI/FS (NCP
§300.515(d)(3jf). EPA must provide the State with an
explanation of any waiver of a State standard
{CERCLA§l21(f)(!)(G}).
For remedial actions under CERCLA §106 thai will
waive an ARAR, the State must be notified at least
30 days prior to the date on which any Consent De-
cree will be entered. If the Slate wishes the action to
conform to (and not waive) ihose standards, the Stale
may intervene in the action before the Consent De-
cree is entered (see § 121(0(2} and (00)).
At certain State-lead sites, the State may make the fi-
nal remedy decision, including a decision to invoke
an ARAR waiver. This situation is restricted to sites
where the State has been assigned the lead role for
[tie response action, the action is being taken under
State law. and the State is not receiving funding for
the action from the Trust Fund. In such situations,
the State may seek, but is not required to obtain, EPA
concurrence on the remedy decision. For further
guidance on this and oilier issues regarding the State
role in remedy selection, see "Questions arid Answers
Aboul the State Rote in Remedy Selection at Non-
Fund-Finuneal Enforcement Sites" (EPA 199 lc).
Post-remedy •Iniplcmenmuon TI decisions may be
made in cases where an outside party or agency sub-
mits comments requesting a TI determination or EPA
determines on its own initiative that a waiver is war-
ranted. The information considered in making such
decisions should include the same types of informa-
tion and analyses discussed for front-end determina-
tions, except that remedy performance data and
analysis also should be provided. This information
must be entered into the site administrative record be-
fore the TI decision can be made and an ARAR
waiver invoked, There are limitatiuns, however, to
ihe requirement that EPA open the administrative
record to new comments, such as an outside party's
request for a TI determination. EPA is not required
to consider comments on the selected remedy unless
the comments contain "significant information not
contained elsewhere in the administrative record file
which substantially supports the need to significantly
alter the response action" (sec NCP §300.825). The
type and amount of information necessary to meet
this requirement (e.g., the length of time a remedy
must be operated prior to a TI evaluation) will be de-
termined by EPA on a site-specific basis.
A modification to a signed ROD invoking a TI
ARAR waiver generally will require a ROD amend-
ment, since a waiver usually will constitute a funda-
mental change in the remedy. A public comment pe-
riod of 30 days is required for an amendment to a
ROD; this period may be extended to 60 days upon
request,25 A public meeting also should be granted
if requested. In the exceptional case where an BSD
is used to invoke a TI ARAR waiver, public notice
and opportunity for comment also shot)Id be pro-
vided. Farther guidance on ROD amendments is
provided in "Guide to Addressing Pre-ROD and
Post-ROD Changes" (EPA 199 Ib) and upcoming re-
visions to "'Guidance on Preparing Superfund Deci-
sion Documents" (expected Fall 1993),
6.1.2 RCRA
TI decisions at RCRA Corrective Action facilities
will be made cither by the EPA Regional Administra-
tor or by the appropriate Smie agency, depending on
the RCRA program authorization status of the Stale.
EPA's goal in the RCRA corrective action program is
to work cooperatively with individual Stales, regard-
loss of thesr authorization status, to promote consis-
tent TI decisions. As in the Superfund program, it is
recommended that the State and EPA notify and con-
sult each other as early as possible regarding sites
where TI determinations may be made. This notifica-
tion and consultation process may be outlined in the
Slaie/liPA .Memorandum of Understanding.
For States authorized lor Hazardous and Solid Waste
Amendments (HSWA) Corrective Action, the Stole
will have primary authority ibr remedy decisions, in-
cluding Tl decisions, EPA will retain authority for
TI determinations in Suites that are not authorized ibr
HSWA corrective action,
At RCRA permitted facilities, implementation of a TI
determination generally would require a Class 3 permit
modification for the purpose, of specifying (alternative)
corrective measures. "l"his process requires a 45-day
notice and comment period, response to comments, and
25 Public notice and opportunity for common! should be provided before an ARAR waiver is granted, regardless of whether an
Explanation uf Significant Differences (ESD5 or ROD amendment is used io invoke the waiver.
24
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public hearing, if requested. At RCRA facilities
conducting corrective action under an order, Tl de-
terminations generally are implemented through the
negotiation of a new order or an amendment 10 an
existing order. This process generally includes a
30- to 45-day pubic comment period and public
hearing, if requested.
6.1.3 Technical Review and Support
Technical support for the Tl evaluation should be
sought as early in the process as possible, preferably
during the initial scoping of the content of the Tl
evaluation. Tl determinations usually will require
expertise from several disciplines, including hydro-
geology, engineering, and risk assessment
Technical staff within the Regions representing these
disciplines should be part of the T! review team.
EPA's Office of Research and Development (QRD)
technical Liaisons and scientists based in the Regions
also may provide assistance 10 program staff. Further
assistance and review may be obtained from the ORD
laboratories involved in the Technical Support
Project, including the R.S. K.err Environmental
Research Laboratory (Ada, OK), the Risk Reduction
and Engineering Laboratory (Cincinnati, OH), the
Environmental Research Laboratory (Athens, GA),
and the Environmental Monitoring Systems
Laboratory (Las Vegas, NV). The director)- of ORD
technical services may be consulted for further
information (EPA 1993c).
General assistance and site-specific consultation on
technical impracticability issues also is available
from EPA headquarters staff. Inquiries should be di-
rected to the appropriate OSWER program office,
6,2 Duration of Tl Decisions
A determination that ground-water restoration is tech-
nically impracticable and the subsequent selection of
an alternative remedial strategy will be subject to fu-
ture review by EPA.
At Supcrfund sites, an alternative remedial strategy
implemented under a CERCLA Tl waiver remains in
effect so long as thai strategy remains protective of
human health and the environment. Protectivcness in
this context encompasses long-term reliability of the
remedy. If the conditions of protectiveness or reliabil-
ity conditions cease to be met, EPA will determine
what additional remedial actions must be, imple-
mented 10 enhance or augment the existing remedy.
EPA shall conduct a full assessment of the protective-
ness of the alternative remedy at least every five
years at any site where contamination remains above
levels that allow for unrestricted use, as required un-
der NCP §30G,43Q(t)C4)C»),
RCRA Tl decisions will be incorporated into facility
permits or enforcement orders and therefore will be
subject to continual oversight and review. Condi-
tions of ilie permit or order involving the Tl decision
or the alternative strategy may be revisited on a peri-
odic basis to ensure proieciiveness. It may be neces-
sary to modify permits or orders to reflect new infor-
mation that becomes available during the remedy
implementation and monitoring period.7* Additional
measures may be. required by EPA 10 ensure the on-
going proicctiveness and reliability of the remedy.
Further, owner/operators of RCRA facilities may be
required by EPA to undertake additional remedial
measures in the future if subsequent advances in re-
mediation technology make auainmeni of media
cleanup standards technically practicable.
The prolectiveness of an alternative, remedial strategy
ai a Superfund site or RCRA facility must be ensured
through u monitoring program designed to detect re-
leases; from containment areas, migration of contami-
nants to water supply wells, or other releases that
would indie ait; a possible fuiiure of one oi' die remedy
components;, EPA may decide to take any further re-
sponse actions necessary to ensure PR) tec live ness at
any time basal upon whether the alternative remedy
is achieving us required performance standards,
Monitoring data, therefore, mu^i be provided to EPA
on a regular basis "o ensure adequate performance of
the alternative remedy, The formal, content, and re-
porting schedule of die monitoring program will be
determined by RPA as pan of ihi- Tl deierraination
and alternative remedy selection process.
26 RCRA Corrective Action Orders that inwqxiiaie Tl decisions should contain language that retains EPA's authority m review
decisions and complete additional site remediation, as necessary.
25
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7.0 References
Anderson, M.P, and W.W. Woe-ssner, 1992,
Groundwgter.MMel.ing, Academic Press, San
Diego.
Cohen, R.M, and J,W. Mercer, 1993, fiJH
Evaluation. C.K. Smolcy, Boca Raton, FL,
EPA, I988a. "Guidance for Conducting Remedial
Investigations and Feasibility Studies Under
CERCLA. Interim Final," OSWER Directive
9355.3-01, EPA/54Q/G-89/004.
EPA, 1988b. "Guidance on Remedial Actions for
Contaminated Ground Water at Superfund Sites,"
EPA/540/G-8S/001
EPA, 19S9a, "Considerations in Ground Water
Remediation at Superfund Sites," OSWER
Directive 9355.4-03.
EPA, I989b. "Evaluation of Ground-Water Extraction
Remedies," EPA/540/2-89/054, Vols. 1-3.
EPA, !989e, "Overview of ARARs, Focus on ARAR
Waivers," OSWER Publication 9234.2-G3/FS,
EPA, 1990a. "Suggested ROD Language for Various
Ground Water Remediation Options," OSWER
Directive 9283,1-03,
EPA, 1996b, "Superfund Removal Procedures - Action
Memorandum Guidance," OSWER Directive
9360.3-01, EPA/54(VP-9G/004,
EPA, 199la. "ARARs Q's & A's: General Policy,
RCRA, CWA, SOW A, Post-ROD Information, and
Contingent Waivers," OSWER Publication 9234,2-
0 I/PS-A,
EPA, 199Ib. "Guide to Addressing Pre-ROD and
Post-ROD Changes," OSWER Publication
9355.3-G2/FS-4,
EPA, 1991 c, "Questions and Answers About the State
Role in Remedy Selection at Non-Fund-Financed
Enforcement Siies," OSWER Directive 9831.9,
EPA, 19914 "Superfund Removal Procedures -
Guidance on die Consideration of ARARs During
Removal Actions," OSWER Publication 9380.3-02.
EPA, I992a. "Considerations in Ground-Water
Remediation at Superfund Sites and RCRA
Facilities - Update," OSWER Directive 9283.1-06.
EPA, 1992b. "Dense Nonaqueous Phase Liquids - A
Workshop Summary, Dallas, Texas, April 16-18,
1991," Office of Research and Development,
EPA/600/R-92/030.
EPA, 1992c. "Estimating Potential for Occurrence of
DNAPL at Superfund Sites," GSWER Publication
9355.4-07/FS.
EPA, I992d. "Evaluation of Ground-Water Extraction
Remedies, Phase II," OSWER Publication 9355,4-
05, Vols. 1-2,
EPA, I992e. "General Methods for Remedial
Operations Performance Evaluations," Office of
Research and Development, EPA/600/R-92/QQ2.
EPA, 19921', "Ground Water Issue; Fundamentals of
Ground-Water Modeling," EPA/540/S-92/005.
EPA, I992g. "Ground-Water Modeling
Compendium," EPA/50Q/B -92/006.
EPA, I993a. "Evaluation of the Likelihood of DNAPL
Presence at NPL Sites," OSWER Publication
9355.4-13, EPA/540/R-93/073.
EPA, I993b. "Guidance on Conducting Non-Time-
Crilical Removal Actions Under CERCLA,"
OSWER Directive 9360.0-32, EPA/540/R-93/057.
EPA, I993c. 'Technical Assistance Directory," Center
for Environmental Research Infonnaiion, Office of
Research and Development." EPA/6CKJ/K-93/006,
Register. Volume 55, No. 46, March 8, 1990,
"National Oil and Hazardous Substances Pollution
Contingency Plan; Final Rule."
Federal Register. Volume 55, No. 145, July 27, 1990.
"Corrective Action for Solid Waste Management
Units at Hazardous Waste Management Facilities;
Proposed Rule."
26
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