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 contaminants 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 transitioned 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-
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 contaminant mass removal?
Correlating Source Removal with Risks. How can progress of source remediation be measured to best correlate with site-specific risks? Recognizing 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 subsurface 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 sustainably. 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 consideration the concerns of stakeholders? Should life cycle assessment become a standard component of the decision process? How can a greater understanding 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 regulatory programs under which hazardous waste is cleaned up to determine the
size and scope of these programs. The Committee sought to determine (1) the number of sites that have not yet reached closure, (2) principal chemicals 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 Storage 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 programs). The metrics and milestones across all these programs differ, making 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 closure. This number is likely to be an underestimate of the extent of contamination 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 dismiss the conclusion that the total number of 126,000 is an underestimate.
No information is available on the total number of sites with contamination 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 exposure. Depending on state closure requirements, USTs are often closed with contamination remaining due to the biodegradability of petroleum hydrocarbons. 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 unrestricted 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 hydrogeological and contaminant perspective. This total represents the sum of the remaining DoD, CERCLA, RCRA, and DOE sites and facilities, based
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 drinking 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 inconsistencies, 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 programs 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.
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 drinking 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 remediation 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 traditional 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) determining the beneficial uses of groundwater; (b) deciding whether a regulatory 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) utilizing 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 administrative 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
“no action” to the risks associated with the implementation of each remedial 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 assessments 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 concerning 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 providing 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 solvents and/or large downgradient dissolved plumes. This includes surfactant flushing, cosolvent flushing, in situ chemical oxidation, pump and treat for hydraulic containment, physical containment, in situ bioremediation, 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 current knowledge regarding performance and limitations of the technologies, identify remaining gaps in knowledge, and provide case studies supporting
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. Furthermore, future improvements in these technologies are likely to be incremental, 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 demonstration) 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 reduction, 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 database, 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 technologies 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, performance metrics, and sustainability information of each type of remedial technology so that there is a trusted source of information for use in the remedial 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
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 considered 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, leaving 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 contamination outright. The following conclusions and recommendations are made.
Implementing institutional controls at complex sites is likely to be difficult. 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 relationships 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 potential exposure pathway of concern, a full understanding of the risks over
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 developments that can aid in the transition from active remediation to more passive 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 processes controlling contaminant mass flux and attenuation along exposure pathways; (2) improved spatio-temporal monitoring of groundwater contamination through better application of conventional monitoring techniques, 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 distribution 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. Management 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
other remediation technologies should be expanded to integrate compound-specific isotope analysis and molecular biological methods with more conventional biogeochemical characterization and groundwater dating methods. The development of molecular and isotopic diagnostic tools has significantly enhanced the ability to evaluate the performance of degradation 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 technologies, improve their performance, and understand any potential broader environmental impacts. A few developing remediation techniques 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 development that will benefit the transition of sites from active remediation 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 modeling 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 pertinent federal agencies will be capable of providing the funding and other support for the fundamental research and development that is necessary
to meet the challenges facing complex sites. A comprehensive assessment of future research needs, undertaken at the federal level and involving coordination 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 standards 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 monitoring, 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 protective 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 levels 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 concentrations 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 performed. The transition assessment evaluates each of the relevant alternatives (remedy modification or replacement, passive or active long-term management) based on the statutory and regulatory remedy selection criteria. 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.
The following conclusions and recommendations about this alternative approach are made.
At many complex sites, contaminant concentrations in the plume remain stalled at levels above cleanup goals despite continued operation of remedial systems. There is no clear path forward to a final end state embodied in the current cleanup programs, such that money continues to be spent, with no concomitant reduction in risks. If the effectiveness of site remediation 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 implemented at CERCLA facilities and could be improved. To increase transparency 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 implemented 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 parties 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, Internet 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 management, 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.