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Research Activities
Research Activities
Groundwater Research
Contaminated groundwater is a cleanup problem at most Superfund sites; virtually all sites with leaking underground storage tanks, and many sites designated for cleanup under the Resource Conservation and Recovery Act (RCRA). It is now known that remediation efforts are taking much longer than originally anticipated. Operation and maintenance costs are substantial and conventional remedies may not be able to achieve cleanup objectives in reasonable time frames or at all, particularly for contaminants that are newly found to pose risks at concentrations once deemed acceptable.
Groundwater research in the Land Research Program (LRP) in EPA’s Office of Research and Development is driven by science and technology questions related to characterizing the contamination and developing cost-effective and efficient remedial options. Research is being conducted to address problematic issues related to contaminated groundwater:
- DNAPLs
- Inorganic species
- Fuel components, including oxygenates
- Vapor Intrusion (contaminants in groundwater)
The LRP is preparing summaries and evaluations of the state of the science to determine the impact of each research area with respect to the long-term stewardship of hazardous waste sites.
Dense Non-Aqueous Phase Liquids (DNAPLs)
The remediation of sites contaminated with dense non-aqueous phase liquids DNAPL is particularly challenging because DNAPLs can migrate deep through the saturated zone, leaving a trail of hydraulically trapped organic liquid. The rates and extent of migration are dependent on physical and chemical properties of the DNAPL and hydrodynamic properties of the subsurface. The resultant spatial distribution of DNAPLs can be very complex and difficult to locate and characterize. The region of the subsurface containing DNAPL, either as randomly distributed pools of organic liquid or sub-zones at residual saturations, is termed the DNAPL source zone.
Constituents from trapped DNAPL slowly partition into groundwater to create and sustain dissolved contaminant plumes that may extend significant distances from the source zone. The extent of the dissolved plume will be determined by contaminant flux from the source and the capacity of the subsurface to attenuate the migrating dissolved contaminant. The most prevalent DNAPLs (halogenated organic solvents, e.g., trichloroethylene and tetrachloroethylene) are not readily degraded and relatively large dissolved plumes of these chemicals are frequently observed.
Characterization of groundwater contamination is a critical issue, particularly for DNAPLs, which do not behave like dissolved contaminants, and for vapor intrusion into buildings which has parameter uncertainty issues for modeling exposure and migration of groundwater contaminants into buildings.
Research will produce characterization, sampling, and analytical methods to reduce the uncertainty in fate and transport models, which will improve exposure estimates supporting risk assessments. Improved understanding of the nature and extent of contamination also will influence selection and implementation of remedial actions and will result in more accurate cost estimates.
Development or enhancement of remediation technologies on challenging groundwater contamination issues is a significant segment of the research effort. Recent research and demonstration projects have led to new alternatives to conventional pump-and-treat technologies that are being adopted in the field.
Conventional remediation techniques designed for dissolved contaminant removal have proven inadequate for achieving acceptable environmental cleanup goals within reasonable time frames for DNAPL source zones. However, field-scale research conducted by the LRP has demonstrated that a high percentage of DNAPL mass can be rapidly depleted from source zones by using aggressive in situ thermal or chemical flushing technologies. Even with these aggressive technologies, the efficiency of DNAPL removal often decays exponentially with increasing mass removed, and complete DNAPL removal may not be technically or economically feasible. For such sites, the key questions are as follows:
How much DNAPL must be removed to be protective of human health and the environment
Are current technologies adequate to achieve this level of removal?
To answer these questions, the LRP is conducting research to understand the relationships between DNAPL mass depletion, contaminant mass flux from the source zone, and dissolved plume properties. DNAPL source remediation research is focused on three critical issues:
- Demonstration, evaluation, and optimization of DNAPL remediation technologies
- Assessment and prediction of the benefits of partial DNAPL depletion
- Development and assessment of integrated DNAPL source remediation approaches.
Inorganic Species
Increasingly, inorganic contamination of groundwater resources from arsenic, chromium, perchlorate, and radionuclides is recognized as a significant issue. Sources include large U.S. Department of Energy and mining mega sites as well as industrial and naturally occurring sources such as arsenic. Remediation of inorganic plumes in groundwater follows a dual track of assessing the potential effectiveness of two techniques:
The LRP’s remediation research builds on the successful development of these approaches for organic contaminants by applying them to inorganic contaminants. Because metals can’t be destroyed, the mechanisms of immobilization are critical to the long-term performance of these approaches.
MNA research focuses on identifying the attenuation mechanisms and the anticipated stability of the immobilized metals under anticipated geochemical conditions. One of the field studies will test the accuracy of the recently completed, cross-office framework for how to evaluate the applicability of an MNA remedy for a specific site.
Research on PRBs will provide two cases of PRB performance in the near-term. Additional research will evaluate the long-term performance and efficiency of PRBs and extend the range of metals that can be addressed with this technology.
Complex hydrogeology impedes the evaluation and remediation of contaminants at sites; additional guidance on effective options for these sites will be provided. Regional experience and research are increasingly identifying the zone of ground water discharge to surface water to be the site of complex interactions. While past research addressed this as it was encountered in field studies, planned research will address the groundwater–surface water interaction (GSI) zone more systematically.
Fuel components, including oxygenates
Research by the LRP on releases from leaking underground fuel storage tanks (LUSTs) will include oxygenates that represent a characterization and remediation challenge in themselves as well as affecting the behavior and treatment of hydrocarbon fuel components. Fate and transport studies will lead to improved modeling capability that predicts plume behavior and the effectiveness of remedial alternatives. Development will continue on a set of online calculators and a modeling system intended to be used by states in assessing the large number of LUST corrective action sites.
Vapor Intrusion
The LRP recently synthesized vapor intrusion research in the document entitled Assessment of Vapor Intrusion in Homes Near the Raymark Superfund Site Using Basement and Sub-Slab Air Samples. The method and associated quality control measures developed for sub-slab sampling are being used at EPA regional offices across the United States. Several states, including California and Colorado, have incorporated many of the report’s recommendations into state guidance documents on vapor intrusion. Further research to improve modeling and sampling methods are being conducted to reduce uncertainty in analysis for vapor intrusion into homes. For example, modeling temporal moisture content variability beneath and external to a building to support potential effects on vapor intrusion risk assessment.