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Summary
At hundreds of thousands of hazardous waste sites across the country,
groundwater contamination remains in place at levels above cleanup goals.
The most problematic sites are those with potentially persistent contami-
nants including chlorinated solvents recalcitrant to biodegradation, and
with hydrogeologic conditions characterized by large spatial heterogeneity
or the presence of fractures. While there have been success stories over the
past 30 years, the majority of hazardous waste sites that have been closed
were relatively simple compared to the remaining caseload. In 2004, the
U.S. Environmental Protection Agency (EPA) estimated that more than
$209 billion would be needed to mitigate these hazards over the next 30
years—likely an underestimate because this number did not include sites
where remediation was already underway or where remediation had tran-
sitioned to long-term management.
The Department of Defense (DoD) exemplifies a responsible party that
has made large financial investments (over $30 billion) in hazardous waste
remediation to address past legacies of their industrial operations. Although
many hazardous waste sites at military facilities have been closed with no
further action required, meeting goals like drinking water standards in
contaminated groundwater has rarely occurred at many complex DoD sites.
It is probable that these sites will require significantly longer remediation
times than originally predicted and, thus, continued financial demands for
monitoring, maintenance, and reporting.
In this context, the Water Science and Technology Board, under the
auspices of the National Research Council (NRC), convened a committee
to assess the future of the nation’s groundwater remediation efforts focus-
1
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2 MANAGING THE NATION’S CONTAMINATED GROUNDWATER SITES
ing on the technical, economic, and institutional challenges facing the Army
and other responsible parties as they pursue site closure. Previous NRC
reports concluded that complete restoration of contaminated groundwater
is unlikely to be achieved for many decades for a substantial number of
sites, in spite of the fact that technologies for removing contaminants from
groundwater have continued to evolve and improve. Since the most recent
NRC report in 2005, better understanding of technical issues and barriers
to achieving site closure have become evident. The following questions
comprised the statement of task for this Committee, which considered both
public and private hazardous waste sites.
Size of the Problem. At how many sites does residual contamination
remain such that site closure is not yet possible? At what percentage of
these sites does residual contamination in groundwater threaten public
water systems?
Current Capabilities to Remove Contamination. What is technically
feasible in terms of removing a certain percentage of the total contaminant
mass? What percent removal would be needed to reach unrestricted use or
to be able to extract and treat groundwater for potable reuse? What should
be the definition of “to the extent practicable” when discussing contami-
nant mass removal?
Correlating Source Removal with Risks. How can progress of source
remediation be measured to best correlate with site-specific risks? Recogniz-
ing the long-term nature of many problems, what near-term endpoints for
remediation might be established? Are there regulatory barriers that make
it impossible to close sites even when the site-specific risk is negligible and
can they be overcome?
The Future of Treatment Technologies. The intractable nature of sub-
surface contamination suggests the need to discourage future contaminant
releases, encourage the use of innovative and multiple technologies, modify
remedies when new information becomes available, and clean up sites sus-
tainably. What progress has been made in these areas and what additional
research is needed?
Better Decision Making. Can adaptive site management lead to better
decisions about how to spend limited resources while taking into consid-
eration the concerns of stakeholders? Should life cycle assessment become
a standard component of the decision process? How can a greater under-
standing of the limited current (but not necessarily future) potential to
restore groundwater be communicated to the public?
MAGNITUDE OF THE PROBLEM
Chapter 2 presents information on the major federal and state regula-
tory programs under which hazardous waste is cleaned up to determine the
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SUMMARY 3
size and scope of these programs. The Committee sought to determine (1)
the number of sites that have not yet reached closure, (2) principal chemi-
cals of concern, (3) remediation costs expended to date, (4) cost estimates
for reaching closure, and (5) the number of sites affecting local water
supplies. Information was gathered for sites in the EPA’s Comprehensive
Environmental Response, Compensation, and Liability Act (CERCLA),
Resource Conservation and Recovery Act (RCRA), and Underground Stor-
age Tank (UST) programs; sites managed by the DoD, the Department of
Energy (DOE) and other federal agencies; and sites under state purview
(e.g., state Superfund, voluntary cleanup programs, and Brownfields pro-
grams). The metrics and milestones across all these programs differ, mak-
ing comparisons and the elimination of overlap difficult. Nonetheless, the
Committee used these data to estimate the number of complex sites, the
likelihood that sites affect a drinking water supply, and the remaining costs
associated with remediation.
At least 126,000 sites across the country have been documented that
have residual contamination at levels preventing them from reaching clo-
sure. This number is likely to be an underestimate of the extent of contami-
nation in the United States for many reasons. For example, the CERCLA
and RCRA programs report the number of facilities, which are likely to
have multiple sites. The total does not include DoD sites that have reached
remedy in place or response complete, although some such sites may indeed
contain residual contamination. Although there is overlap between some of
the categories, in the Committee’s opinion it is not significant enough to dis-
miss the conclusion that the total number of 126,000 is an underestimate.
No information is available on the total number of sites with contami-
nation in place above levels allowing for unlimited use and unrestricted
exposure, although the total is certainly greater than 126,000. For the
CERCLA program, many facilities have been delisted with contamination
remaining in place at levels above unlimited use and unrestricted expo-
sure. Depending on state closure requirements, USTs are often closed with
contamination remaining due to the biodegradability of petroleum hydro-
carbons. Most of the DOE sites, including those labeled as “completed,”
contain recalcitrant contamination that in some cases could take hundreds
of years to reach levels below those allowing for unlimited use and unre-
stricted exposure.
A small percentage (about 12,000 or less than 10 percent) of the
126,000 sites are estimated by the Committee to be complex from a hydro-
geological and contaminant perspective. This total represents the sum of
the remaining DoD, CERCLA, RCRA, and DOE sites and facilities, based
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4 MANAGING THE NATION’S CONTAMINATED GROUNDWATER SITES
on the assumption that many of the simpler sites in these programs have
already been dealt with.
Approximately 10 percent of CERCLA facilities affect or significantly
threaten public water supply systems, but similar information from other
programs is largely unavailable. Surveys of groundwater quality report
that 0.34 to 1 percent of raw water samples from wells used for drinking
water (including public supply and private wells) contain mean volatile
organic compound (VOC) concentrations greater than the applicable drink-
ing water standard, although there are no data linking these exceedances to
specific hazardous waste sites. The percentage of drinking water wells with
samples containing low-level VOC concentrations is likely to be higher for
areas in close proximity to contaminated sites, for urban rather than rural
areas, and in shallow unconfined sandy aquifers.
Information on cleanup costs incurred to date and estimates of future
costs are highly uncertain. Despite this uncertainty, the estimated “cost to
complete” of $110-127 billion is likely to be an underestimate of future
liabilities. Remaining sites include some of the most difficult to remediate
sites, for which the effectiveness of planned remediation remains uncertain
given their complex site conditions. Furthermore, many of the estimated
costs do not fully consider the cost of long-term management of sites that
will have contamination remaining in place at levels above those allowing
for unlimited use and unrestricted exposure for the foreseeable future.
The nomenclature for the phases of site cleanup and cleanup progress
are inconsistent between federal agencies, between the states and federal
government, and in the private sector. Partly because of these inconsisten-
cies, members of the public and other stakeholders can and have confused
the concept of “site closure” with achieving unlimited use and unrestricted
exposure goals for the site, such that no further monitoring or oversight
is needed. In fact, many sites thought of as “closed” and considered as
“successes” will require oversight and funding for decades and in some
cases hundreds of years in order to be protective. CERCLA and other pro-
grams have reduced public health risk from groundwater contamination
by preventing unacceptable exposures in water or air, but not necessarily
by reducing contamination levels to drinking water standards throughout
the affected aquifers.
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SUMMARY 5
REMEDIAL OBJECTIVES, REMEDY
SELECTION, AND SITE CLOSURE
Chapter 3 focuses on the remedial objectives dictated by the common
regulatory frameworks under which groundwater cleanup generally occurs
because such objectives are often a substantial source of controversy. This is
particularly true for complex sites, where the remedial objectives are drink-
ing water standards (denoted as maximum contaminant levels or MCLs)
and hence are typically difficult, if not impossible, to attain for many
decades. Faced with shrinking budgets and a backlog of sites that include
an increasing percentage of complex sites, some states (e.g., California)
have proposed closing large numbers of petroleum underground storage
tank sites deemed to present a low threat to the public, despite the affected
groundwater not meeting remedial goals at the time of closure. Other states
(New Jersey and Massachusetts) have sought to privatize parts of the re-
mediation process in order to unburden state and local regulatory agencies.
EPA’s current remediation guidance provides substantial flexibility to
the remedy selection process in a number of ways, although there are legal
and practical limits to this flexibility. There are several alternatives to tradi-
tional cleanup goals, like technical impracticability waivers, that can allow
sites with intractable contamination to move more expeditiously through
the phases of cleanup while still minimizing risks to human health and the
environment. The chapter also discusses sustainability concepts, which have
become goals for some stakeholders and could impact the remedy selection
process. The following conclusions and recommendations discuss the value
of exploring goals and remedies based on site-specific risk, sustainability,
and other factors.
By design (and necessity), the CERCLA process is flexible in (a) deter-
mining the beneficial uses of groundwater; (b) deciding whether a regula-
tory requirement is an applicable or relevant and appropriate requirement
(ARAR) at a site; (c) using site-specific risk assessment to help select the
remedy; (d) using at least some sustainability factors to help select the
remedy; (e) determining what is a reasonable timeframe to reach remedial
goals; (f) choosing the point of compliance for monitoring; and (g) utiliz-
ing alternate concentration limits, among others. These flexible approaches
to setting remedial objectives and selecting remedies should be explored
more fully by state and federal regulators, and EPA should take admin-
istrative steps to ensure that existing guidance is used in the appropriate
circumstances.
To fully account for risks that may change over time, risk assessment
at contaminated groundwater sites should compare the risks from taking
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6 MANAGING THE NATION’S CONTAMINATED GROUNDWATER SITES
“no action” to the risks associated with the implementation of each reme-
dial alternative over the life of the remedy. Risk assessment at complicated
groundwater sites is often construed relatively narrowly, with an emphasis
on risks from drinking water consumption and on the MCL. Risk assess-
ments should include additional consideration of (a) short-term risks that
are a consequence of remediation; (b) the change in residual risk over time;
(c) the potential change in risk caused by future changes in land use; and
(d) both individual and population risks.
Progress has been made in developing criteria and guidance concern-
ing how to consider sustainability in remedy selection. However, in the
absence of statutory changes, remedy selection at private sites regulated
under CERCLA cannot consider the social factors, and may not include
the other economic factors, that fall under the definition of sustainability.
At federal facility sites, the federal government can choose, as a matter of
policy, to embrace sustainability concepts more comprehensively. Similarly,
private companies may adopt their own sustainable remediation policies in
deciding which remedial alternatives to support at their sites. New guidance
is needed from EPA and DoD detailing how to consider sustainability in
the remediation process to the extent supported by existing laws, including
measures that regulators can take to provide incentives to companies to
adopt more sustainable measures voluntarily.
CURRENT CAPABILITIES TO
REMOVE/CONTAIN CONTAMINATION
Chapter 4 updates the 2005 NRC report on source removal by provid-
ing brief reviews of the major remedial technologies that can be applied
to complex hazardous waste sites, particularly those with source zones
containing dense nonaqueous phase liquids (NAPLs) like chlorinated sol-
vents and/or large downgradient dissolved plumes. This includes surfac-
tant flushing, cosolvent flushing, in situ chemical oxidation, pump and
treat for hydraulic containment, physical containment, in situ bioreme-
diation, permeable reactive barriers, and monitored natural attenuation.
Well-established technologies including excavation, soil vapor extraction/
air sparging, and solidification/stabilization are not discussed because they
have been presented in prior publications and minimal advancements in
these technologies have occurred over the past five to ten years. To address
what is technically feasible in terms of removing a certain percentage of
the total contaminant mass from the subsurface, the sections discuss cur-
rent knowledge regarding performance and limitations of the technologies,
identify remaining gaps in knowledge, and provide case studies supporting
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SUMMARY 7
these assessments. The following conclusions and recommendations arise
from this chapter.
Significant limitations with currently available remedial technologies
persist that make achievement of MCLs throughout the aquifer unlikely
at most complex groundwater sites in a time frame of 50-100 years. Fur-
thermore, future improvements in these technologies are likely to be in-
cremental, such that long-term monitoring and stewardship at sites with
groundwater contamination should be expected.
The Committee could identify only limited data upon which to base a
scientifically supportable comparison of remedial technology performance
for the technologies reviewed in Chapter 4. There have been a few well-
studied demonstration projects and lab-scale research studies, but adequate
performance documentation generated throughout the remedial history at
sites either is not available or does not exist for the majority of completed
remediation efforts. Furthermore, poor design, poor application, and/or
poor post-application monitoring at typical (i.e., non-research or dem-
onstration) sites makes determination of the best practicably achievable
performance difficult.
There is a clear need for publically accessible databases that could be
used to compare the performance of remedial technologies at complex sites
(performance data could be concentration reduction, mass discharge reduc-
tion, cost, time to attain drinking water standards, etc.). To ensure that data
from different sites can be pooled to increase the statistical power of the da-
tabase, a standardized technical protocol would be needed, although it goes
beyond the scope of this report to provide the details of such a protocol.
Additional independent reviews of source zone technologies are needed
to summarize their performance under a wide range of site characteristics.
Since NRC (2005), only thermal and in situ chemical oxidation technolo-
gies have undergone a thorough, independent review. Other source zone
technologies should also be reviewed by an independent scientific group.
Such reviews should include a description of the state of the practice, per-
formance metrics, and sustainability information of each type of remedial
technology so that there is a trusted source of information for use in the re-
medial investigation/feasibility study process and optimization evaluations.
IMPLICATIONS OF CONTAMINATION REMAINING IN PLACE
Chapter 5 discusses the potential technical, legal, economic, and other
practical implications of the finding that groundwater at complex sites is
unlikely to attain unlimited use and unrestricted exposure levels for many
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8 MANAGING THE NATION’S CONTAMINATED GROUNDWATER SITES
decades. First, the failure of hydraulic or physical containment systems, as
well as the failure of institutional controls, could create new exposures.
Second, toxicity information is regularly updated, which can alter drinking
water standards, and contaminants that were previously unregulated may
become so. In addition, pathways of exposure that were not previously con-
sidered can be found to be important, such as the vapor intrusion pathway.
Third, treating contaminated groundwater for drinking water purposes is
costly and, for some contaminants, technically challenging. Finally, leav-
ing contamination in the subsurface may expose the landowner, property
manager, or original disposer to complications that would not exist in the
absence of the contamination, such as natural resource damages, trespass,
and changes in land values. Thus, the risks and the technical, economic,
and legal complications associated with residual contamination need to be
compared to the time, cost, and feasibility involved in removing contamina-
tion outright. The following conclusions and recommendations are made.
Implementing institutional controls at complex sites is likely to be dif-
ficult. Although EPA has developed a number of measures to improve the
reliability, enforceability, and funding of institutional controls, their long-
term efficacy has yet to be determined. Regulators and federal responsible
parties should incorporate a more significant role for local citizens in
the long-term oversight of institutional controls. A national, searchable,
geo-referenced institutional control database covering as many regulatory
programs as practical as well as all federal sites would help ensure that the
public is notified of institutional controls.
New toxicological understanding and revisions to dose-response re-
lationships will continue to be developed for existing chemicals, such as
trichloroethene and tetrachloroethene, and for new chemicals of concern,
such as perchlorate and perfluorinated chemicals. The implications of such
evolving understanding include identification of new or revised ARARs
(either more or less restrictive than existing ones), potentially leading to a
determination that the existing remedy at some hazardous waste sites is no
longer protective of human health and the environment. Modification of
EPA’s existing CERCLA five-year review guidance would allow for more
expeditious assessment of the protectiveness of the remedy based on any
changes in EPA toxicity factors, drinking water standards, or other risk-
based standards.
Careful consideration of the vapor intrusion pathway is needed at all
sites where VOCs are present in the soil or groundwater aquifer. Although
it has been recognized for more than a decade that vapor intrusion is a po-
tential exposure pathway of concern, a full understanding of the risks over
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SUMMARY 9
time and appropriate methods for characterizing them are still evolving.
Mitigation strategies such as subslab depressurization can prevent vapor
intrusion exposure. As a precautionary measure, vapor mitigation could
be built into all new construction on or near known VOC groundwater
plumes. Vapor mitigation systems require monitoring over the long term
to ensure that they are operating properly.
TECHNOLOGY DEVELOPMENT TO SUPPORT
LONG-TERM MANAGEMENT
Despite years of characterization and implementation of remedial
technologies, many complex federal and private industrial facilities with
contaminated groundwater will require long-term management that could
extend for decades or longer. Chapter 6 discusses technological develop-
ments that can aid in the transition from active remediation to more pas-
sive strategies and provide more cost-effective and protective long-term
management of complex sites. In particular, transitioning to and improving
long-term management can be achieved through (1) better understanding
of the spatial distribution of contaminants, exposure pathways, and pro-
cesses controlling contaminant mass flux and attenuation along exposure
pathways; (2) improved spatio-temporal monitoring of groundwater con-
tamination through better application of conventional monitoring tech-
niques, the use of proxy measurements, and development of sensors; and
(3) application of emerging diagnostic and modeling tools. The chapter
also explores emerging remediation technologies that have yet to receive
extensive field testing and evaluation, and it reviews the state of federal
funding for relevant research and development. The following conclusions
and recommendations are offered.
Long-term management of complex sites requires an appropriately
detailed understanding of geologic complexity and the potential distri-
bution of contaminants among the aqueous, vapor, sorbed, and NAPL
phases, as well as the unique biogeochemical dynamics associated with
both the source area and downgradient plume. Recent improvements to
the understanding of subsurface biogeochemical processes have not been
accompanied by cost-effective site characterization methods capable of
fully distinguishing between different contaminant compartments. Manage-
ment of residual contamination to reduce the exposure risks via the vapor
intrusion pathway is challenged by the highly variable nature of exposure,
as well as uncertain interactions between subsurface sources and indoor
background contamination.
Existing protocols for assessing monitored natural attenuation and
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10 MANAGING THE NATION’S CONTAMINATED GROUNDWATER SITES
other remediation technologies should be expanded to integrate com-
pound-specific isotope analysis and molecular biological methods with
more conventional biogeochemical characterization and groundwater dat-
ing methods. The development of molecular and isotopic diagnostic tools
has significantly enhanced the ability to evaluate the performance of deg-
radation technologies and monitored natural attenuation at complex sites.
Although the Committee did not attempt a comprehensive assessment
of research needs, research in the following areas would help address
technical challenges associated with long-term management at complex
contaminated sites (see Chapter 6 for a more complete list):
• Remediation Technology Development. Additional work is needed
to advance the development of emerging and novel remediation tech-
nologies, improve their performance, and understand any potential
broader environmental impacts. A few developing remediation tech-
niques could provide more cost-effective remediation for particular
combinations of contaminants and site conditions at complex sites,
but they are in the early stages of development.
• Tools to Assess Vapor Intrusion. Further research and development
should identify, test, and demonstrate tools and paradigms that
are practicable for assessing the significance of vapor intrusion,
especially for multi-building sites and preferably through short-
term diagnostic tests. Development of real-time unobtrusive and
low-cost air quality sensors would allow verification of those short-
term results over longer times at buildings not needing immediate
mitigation.
• Modeling. Additional targeted modeling research and software de-
velopment that will benefit the transition of sites from active reme-
diation to long-term management should be initiated. Particular
needs include concepts and algorithms for including the processes of
back-diffusion and desorption in screening and plume models, and
the development of a larger suite of intermediate-complexity model-
ing tools to support engineering design for source remediation.
Overall research and development have been unable to keep pace with
the needs of practitioners trying to conduct remediation on complex sites.
Currently, a national strategy for technology development to support long-
term management of complex sites is lacking. It is not clear that the per-
tinent federal agencies will be capable of providing the funding and other
support for the fundamental research and development that is necessary
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SUMMARY 11
to meet the challenges facing complex sites. A comprehensive assessment
of future research needs, undertaken at the federal level and involving co-
ordination between federal agencies, would allow research funding to be
allocated in an efficient and targeted manner.
BETTER DECISION MAKING DURING THE
LONG-TERM MANAGEMENT OF COMPLEX
GROUNDWATER CONTAMINATION SITES
The fact that at most complex groundwater sites drinking water stan-
dards will not be attained for decades should be more fully reflected in the
decision-making process of existing cleanup programs. Thus, Chapter 7
provides a series of recommendations that will accelerate the transition of
sites to one of three possible end states: (1) closure in which unlimited use
and unrestricted exposure levels have been attained; (2) long-term passive
management (e.g., using natural attenuation with or without monitor-
ing, physical containment, permeable reactive barriers, and/or institutional
controls), and (3) long-term active management (e.g., indefinite hydraulic
containment using pump and treat). The acceleration of this transition to
one of three end states is premised on using remedies that are fully protec-
tive of human health and the environment in combination with more rapid
acceptance of alternative end states other than clean closure.
An alternative approach for better decision making at complex sites
is shown in Figure 7-2. It includes the processes currently followed at all
CERCLA facilities and at many complex sites regulated under other federal
or state programs (RCRA or state Superfund), but it provides more detailed
guidance for sites where recalcitrant contamination remains in place at lev-
els above those allowing for unlimited use and unrestricted exposure. This
alternative approach diverges from the status quo by requiring the explicit
charting of risk reduction (as indicated by, e.g., contaminant concentration
reduction) over time. Specifically, if data indicate that contaminant concen-
trations are approaching an asymptote, resulting in exponential increases
in the unit cost of the remedy, then there is limited benefit in its continued
operation. At this point of diminishing returns, it is appropriate to assess
whether to take additional remedial action (if legally possible) or whether
to transition to more passive long-term management.
If asymptotic conditions have occurred, a transition assessment is per-
formed. The transition assessment evaluates each of the relevant alterna-
tives (remedy modification or replacement, passive or active long-term
management) based on the statutory and regulatory remedy selection cri-
teria. This includes consideration of the risk from residual contamination
in subsurface zones, life-cycle costs and the incremental costs compared to
the level of risk reduction achieved, and the likely reaction of stakeholders.
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12 MANAGING THE NATION’S CONTAMINATED GROUNDWATER SITES
The following conclusions and recommendations about this alternative ap-
proach are made.
At many complex sites, contaminant concentrations in the plume re-
main stalled at levels above cleanup goals despite continued operation of
remedial systems. There is no clear path forward to a final end state em-
bodied in the current cleanup programs, such that money continues to be
spent, with no concomitant reduction in risks. If the effectiveness of site re-
mediation reaches a point of diminishing returns prior to reaching cleanup
goals and optimization has been exhausted, the transition to monitored
natural attenuation or some other active or passive management should
be considered using a formal evaluation. This transition assessment would
determine whether a new remedy is warranted at the site or whether long-
term management is appropriate.
Five-year reviews are an extremely valuable source of field data for
evaluating the performance of remedial strategies that have been imple-
mented at CERCLA facilities and could be improved. To increase trans-
parency and allow EPA, the public, and other researchers to assess lessons
learned, more should be done, on a national basis, to analyze the results
of five-year reviews in order to evaluate the current performance of imple-
mented technologies. EPA’s technical guidance for five-year reviews should
be updated to provide a uniform protocol for analyzing the data collected
during the reviews, reporting their results, and improving their quality.
Public involvement tends to diminish once remedies at a site or facility
are in place. No agency has a clear policy for sustaining public involvement
during long-term management. Regulators and federal responsible par-
ties should work with members of existing advisory groups and technical
assistance recipients to devise models for ongoing public oversight once
remedies are in place. Such mechanisms may include annual meetings, In-
ternet communications, or the shifting of the locus of public involvement
to permanent local institutions such as public health departments.
Although the cost of new remedial actions may decrease at complex
sites if more of them undergo a transition to passive long-term manage-
ment, there will still be substantial long-term funding obligations. Failure
to fund adequately the long-term management of complex sites may result
in unacceptable risks to the public due to unintended exposure to site
contaminants.
