Vapor Intrusion from Entrapped NAPL Sources and Groundwater Plumes: Process Understanding and Improved Modeling Tools for Pathway Assessment
ER-1687
Objective
Vapor intrusion (VI) refers to the transport of volatile chemical vapors from subsurface sources to surface and subsurface structures. Volatile organic compounds (VOC) are commonly found entrapped as non-aqueous phase liquids (NAPL) in the soil pores or dissolved in groundwater at industrial waste sites, refineries, and Department of Energy (DOE) and Department of Defense (DoD) complexes. Vapors emitted from these contaminant sources readily disperse into the atmosphere and air-filled void spaces within the soil and migrate below surface structures, leading to the intrusion of contaminant vapors into indoor air. Screening models are often used to examine potential exposure pathways and to assess VI risk. These modeling tools, which rely on limited data, play an important role in regulatory action and decision making; however, the limitations of these models are well understood. Assessment of model limitations when applied to complex situations (e.g., source, subsurface heterogeneities and soil types, building configurations, and boundary conditions at the land/atmospheric interface) is essential for their proper application. The focus of this research is a comprehensive study of vapor generation and vapor intrusion, using carefully controlled experiments in multiscale test systems that include a large porous media tank coupled with a climate wind tunnel designed to reproduce realistic boundary conditions at the land/atmospheric interface. Results from this work will identify limitations in the application of simplified and complex screening models, propose alternative formulations, and generally aid remediation site managers with the assessment of risk and the selection of alternative remediation strategies.
The main objective of this project is an improved understanding of the processes and mechanisms controlling vapor generation from entrapped NAPL sources and groundwater plumes, their subsequent migration through the subsurface, and their attenuation in naturally heterogeneous vadose zones under various natural physical, climatic, and geochemical conditions. Experiments conducted at multiple scales will be integrated with analytical and numerical modeling and field data to test and validate existing VI theories and models.
Technical Approach
The basic hypothesis is that significant error in the prediction of indoor air quality is likely due to a lack of understanding of the influence of factors such as subsurface heterogeneity, soil moisture, contaminant attenuation, and the interaction of the subsurface with the atmosphere (i.e., atmospheric boundary conditions) on vapor intrusion. Testing this hypothesis will require an investigation of scientific issues related to (i) vapor generation from entrapped NAPL sources under various source and water table conditions, (ii) interphase partitioning of contaminants between dissolved plumes and the gas phase in porous media, and (iii) subsequent vapor migration and attenuation, which is controlled by biogeochemical processes. Insights gained through this work may be used to validate VI modeling methods and guide improvements to existing screening type models, ultimately suggesting better strategies to conduct pathway analysis and characterization at VOC-contaminated DoD and DOE sites. The end product of this research will be a comprehensive decision making protocol and guidelines that can aid in site management and risk analysis of VI pathway-related problems.
Benefits
This research will improve the scientific understanding that forms the foundation for site management and decision making, guide improvements to existing models to better perform pathway analysis, and identify promising cost-effective site characterization methods. Benefits to DoD include improved management guidance for VI sites, ultimately leading to a reduction in risk to subsurface structures and homes plagued by indoor air pollution issues, and more informed regulatory decision making on contaminant cleanup levels. These improvements will, in effect, reduce human health concerns and liability. Additionally, the findings will eliminate current knowledge gaps in the understanding of complex vapor generation, migration, and attenuation occurring under natural climatic conditions at the land/atmospheric interface. (Anticipated Project Completion - 2012)
Interim Results
Although work continues, preliminary results are confirming the hypothesis that vapor migration in the subsurface is a complex process controlled by dynamic interaction at the land-atmosphere interface. Intermediate scale experiments combined with modeling analyses are demonstrating the impact surface precipitation events and thermal boundary conditions can have on the migration pathway of soil vapors. Work is on-going to use the data to validate models that are able to predict the development of dynamic vapor pathways driven by climate. Preliminary data in small test cells and tanks suggest that the process of vapor generation from both entrapped NAPL sources and dissolved constituents in the groundwater plume may be rate limited, potentially indicating that assumptions based on equilibrium models may over-predict subsurface concentrations and under-predict source longevity.
Points of Contact
Principal Investigator
Dr. Tissa Illangasekare
Colorado School of Mines
Phone: 303-384-2126
Fax: 303-273-3311
Document Types
- Fact Sheet - Brief project summary with links to related documents and points of contact.
- Final Report - Comprehensive report for every completed SERDP and ESTCP project that contains all technical results.
- Cost & Performance Report - Overview of ESTCP demonstration activities, results, and conclusions, standardized to facilitate implementation decisions.
- Technical Report - Additional interim reports, laboratory reports, demonstration reports, and technology survey reports.
- Guidance - Instructional information on technical topics such as protocols and user’s guides.
- Workshop Report - Summary of workshop discussion and findings.
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- Model/Software - Computer programs and applications available for download.
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