VAPOR INTRUSION ASSESSMENT

Using the USEPA Flux Chamber to Directly Measure Open Soil and Slab Infiltration on Vapor Intrusion Sites Supporting the Site Migration Pathway Analysis

There isn't a challenge greater today for an atmospheric scientist than assessing sites that have the potential for vapor intrusion and exposure to occupants to hazardous compounds.   The mechanism is the transport of vapor transport from a subsurface source, like contaminated groundwater or contaminated soil (typically hydrocarbon compounds) through the soil column into a structure.  The driving force from the source is vapor pressure of the compound and diffusion, but gases can also move by pressure gradients caused by wind, thermal gradients, and building under-pressurization.  Vapor migration and intrusion into a structure is influenced by many factors, which makes the task of exposure assessment very difficult.  Guidance documents promote a flow-chart process of screening and in-depth assessments.  The process typically includes the study of the site history and assessment of the existence of the contamination followed by field assessment of the potential exposure.  Available technical approaches for this task include soil gas sampling and predictive modeling, sub-slab sampling and predictive modeling, open soil flux chamber sampling (photo 1) and estimation, on-slab indoor chamber sampling (photo 2) and estimation, and indoor (photo 3) and outdoor (photo 4) ambient air sample collection and comparison to risk-based air concentration criteria.

You might think that these sites could easily be studied just by conducting indoor air testing, but that is not the case.  Indoor air (photo 5) is made up of outdoor air, complete with hydrocarbon compounds and off-gassing of hydrocarbons from products, activities, and building materials in the structure.  Divorced from any subsurface contamination and infiltration, indoor air is usually more 'contaminated' than outdoor air.  Couple this limitation with building use and ventilation, indoor air testing does not singularly provide the answers.

The other assessment approaches fall into two categories: assessing the source gas and conducting vapor migration monitoring, or directly measuring open soil flux or vapor intrusion flux.  Soil gas sampling and modeling is popular but conservative, mainly because of the predictive modeling needed to perform the assessment.  Since the USEPA was designed and validated for the purpose of directly measuring flux at the land surface, this approach is superior to any that requires modeling.  Surface flux testing can be performed on the open soil (photo 6) over the subsurface contamination and these data can be used to estimate by potential intrusion.  Flux measurements can also be made on cracks and seams in patios and sidewalks outside (photo 7) and cement slabs inside (photo 8).  The condition of building under-pressurization (photo 9) can also be addressed using the USEPA flux chamber technology.  More information on the advantages and disadvantages of these technologies is given below.  Typically, more than one assessment approach is needed in order to assess potential infiltration of vapors from a subsurface source, and the USEPA flux chamber technology is most useful for conducting these assessments as part of the exposure assessment supporting health risk assessment to building occupants.

     Photo 1 - USEPA Flux Chamber Measuring Open Soil Flux alongside Building Foundation
Photo 2- Indoor Flux Measurement of Vapor Intrusion Through Seams and Cracks in Tile on Slab

Photo 3- Indoor Vapor Intrusion Assessment Including Flux on Seams, Cracks, and Continuous Slab with Concurrent Indoor Ambient Air Testing
Photo 4- Outdoor Open Soil Flux Testing of Chlorinated VOC Emissions and Outdoor Ambient Air Testing as Part of the APA

 
Photo 5- Indoor Air Monitoring Collocated with Infiltration Flux Assessment
Photo 6- USEPA Flux Chamber Testing Over Subsurface Source

 
Photo 7- Infiltration Flux Testing on Outdoor Slab Simulating Indoor Slab
Photo 8- Infiltration Flux Testing on Industrial Slab

Photo 9- Infiltration Flux Testing Under Negative Pressure Conditions
Using Negative Pressure Enclosure

 
Comments on "Draft Guidance For Evaluating The Vapor Intrusion to Indoor Air Pathway From Groundwater and Soils (Subsurface Vapor Intrusion Guidance)"

Docket ID No. RCRA-2002-0033

Submitted to:

USEPA

Submitted by:

CE Schmidt, Ph.D.

19200 Live Oak Road

Red Bluff , CA 96080

(530) 529-4256

Fax- 529-4878

Email: This email address is being protected from spambots. You need JavaScript enabled to view it.

Website: ceschmidt.com

Submitted to:

EPA's Electronic Public Docket

Email to Henry Schuver

 
INTRODUCTION

The following comments on the Draft Guidance For Evaluating The Vapor Intrusion to Indoor Air Pathway From Groundwater and Soil (Subsurface Vapor Intrusion Guidance - Draft Guidance), Document ID RCRA-2002-0033-0001 are submitted to the Federal Register Notice of November 29, 2002.

The Draft Guidance document provides users' with a tool for conducting screening evaluations as to whether or not the air pathway for subsurface vapor intrusion exposure pathway is complete and occupants or residents are potentially exposed to contaminants via infiltration into structures and contamination of indoor air. A three tiered approach including Primary Screening (Tier 1), Secondary Screening (Tier 2), and Site-Specific Screening (Tier 3) is provided that relies on representative site characterization data (soil, groundwater, soil gas concentration data, distance to contamination, and site geology information, etc) and heuristic, predictive modeling to estimate potential exposure from a known subsurface contamination source. The Johnson and Ettinger Model (J&M) is the predictive model used, which has been shown to be conservative screening tool, if used properly, to represent indoor exposure. 1 If site characterization and descriptive data are used properly in the predictive screening tool and the result of the analysis shows that the vapor intrusion pathway is "incomplete", the general consensus is that occupants or residents are not at risk and the exposure potential issue is adequately addressed and dismissed. But if the site is considered by screening to have a "complete" vapor intrusion pathway, then further consideration of the current site situation is recommended. The guidance offers a progression of more complex or more in-depth assessment recommendations that hopefully result in an adequate characterization of potential health risk for occupants/residents in the subject structure.

The focus of these comments is related to those recommendations for sites that have demonstrated a potential vapor intrusion pathway; those sites that have predicted endpoint indoor air concentrations that exceed guidance criteria for acceptable health risk based on inhalation of indoor affected by vapor intrusion. Insufficient technical guidance has been provided to assist the user in conducting direct measurement of exposure potential in support of an adequate health risk assessment needed to establish an incremental health risk to occupants/residents where screening criteria are exceeded.

 
EVALUATION PROCESS RECOMMENDED IN GUIDANCE

The multi-tiered guidance describes a logical progression of site investigation activities that culminate in Tier 3- Site Specific Pathway Assessments (summarized in Figure13 and described on pages 38-50). The guidance relies conceptually on the protocol provided in the USEPA Air/Superfund National Technical Guidance Study Series, specifically Volume 2, whose intent was to provide site managers with the technical guidance for conducting an assessment of the air pathway (air pathway assessment-APA) and developing input for health risk assessment. 2, 3 The focus is on using applicable technologies, both predictive and measured, in order to assess the migration of contaminants from sources to receptors.

Four recommendations are provided for those Tier 3 sites where a complete vapor intrusion pathway is detected, including:

1. Direct measurement of foundation air concentrations before any indoor air measurements;

2. Direct measurement of indoor air concentrations coupled with home survey and sampling to identify background sources of vapor in ambient (outdoor) and/indoor air;

3. Removal of all indoor air sources before sampling indoors; and

4. Complementary site-specific mathematical modeling as appropriate.

Further discussions suggest that additional soil gas testing (e.g. sub slab or crawl space) and mathematical modeling (similar to Tier 2 activities) are recommended given the complexities of direct measurement approaches. Air monitoring of indoor and outdoor air is suggested if the building is accessible and other "more direct measures of potential impact, such as emission flux chambers or soil gas surveys" are recommended for sites, especially if the building is not accessible or doesn't exist (future building scenario), that indicate unacceptable inhalation risk from Tier 1, 2, and 3 screening activities.

Once a potential inhalation risk is identified using appropriate screening level technologies, the most commonly used being soil gas testing and predictive modeling using either the Guidance Document look-up tables or site-specific screening modeling (J&E), there are two direct measurement technologies or assessment approaches that are generally available and applicable that can be used to adequately assess actual, measured exposure to occupants to existing structures, namely: 1) indoor/outdoor ambient air monitoring, and 2) direct measurement of flux from the subsurface both on open soil over the plume and near the structure and, if possible, direct measurement of flux through the slab or sub floor. For the future building scenario site, the single option is direct measurement of flux over the impacted area.

Ambient Air Monitoring

Air monitoring of indoor and outdoor air requires knowledge of sample collection and selection of proper analytical techniques. There are many useful guidance documents available to assist in the direct measurement of indoor/outdoor air. 4, 5, 6 The components of a useful and practical air monitoring program include the selection of proper sample collection and analytical techniques(e.g., USEPA Method TO-14/TO-15 evacuated canisters collected as integrated ambient air samples over 8-to-24 hour time periods and analyzed by selective ion mode GC/MS for sub-ppbv method detection limits of site-specific VOCs), adequate project quality assurance protocols, and a technical approach designed to meet the specific needs and sample collection requirements of the subject site. Basically, ambient air samples are collected as integrated samples in multiple, first floor (or basement) rooms of the study structure, typically at 5-feet above the ground, at multiple locations, and on multiple days. This is required because ambient air data are antidotal and a significant range of ambient air concentration difference is expected as related to structure use and ventilation, assuming a fixed infiltration source. Representative outdoor air is concurrently sampled, along with the collection of site specific or regional ambient air data. Indoor room surveys are also conducted documenting materials or products that may be sources of VOCs beyond typical construction materials or furnishings, and ventilation system operations are documented. In some instances, it may be required to conduct control structure air monitoring (e.g., similar structure but not affected by the subsurface contamination). The complications of VOCs found in outdoor air, and from indoor sources that may range from industrial or manufacturing process or off gassing from materials or furnishings can be handled by studying those compounds as related to the subsurface source individually or as a ratio of study compounds (e.g., fingerprint approach). Even so, studying exposure by using indoor/outdoor air monitoring technologies is not necessarily straightforward. It is, however, an acceptable 'next step' when continued soil gas data (outside soil gas and sub slab soil gas) testing and mathematical models continue to show an unacceptable health risk. If designed and conducted properly, even in the absence of the 'removal of all indoor air source before sampling occurs', is a viable approach to reaching conclusions regarding unacceptable vapor intrusion.

Note that most structures over subsurface sources have some measurable vapor intrusion through slab seams and cracks, and vapors have also been reported migrating through sub flooring even with ventilated crawl spaces (e.g., pier and post construction). However, the goal is to accurately assess the level of infiltration and thus the resulting endpoint ambient air (indoor) concentration. As such, a source assessment approach is desirable.

Direct Measurement of Area Source Flux

The other direct measurement approach referenced in the Guidance Document as a Tier 3 approach is flux chambers. The USEPA has provided a Surface Isolation Flux Chamber Guidance Document that describes the construction and use of a dynamic flux chamber that has application as a 'source assessment' technology for studying vapor intrusion into structures 7 . The technology is described as an 'in-depth' assessment technology in the Technical Guidance Series Volume 2 and other guidance documents for collecting direct measured flux data from area sources. 2, 3 The technology provides for the collection of volatile and semi-volatile compound flux from solid, sludge, and liquid surfaces at atmospheric pressure conditions and has been reported as an effective technology for generating data of known accuracy and precision from area sources. 8-14 Initial applications of the technology focused on RCRA treatment, storage and disposal air pathway analysis and CERCLA uncontrolled waste site RI/FS project support including controlling (engineering analysis) and predicting off-site impacts from site activities such as excavation and in-situ or on-site waste treatment. 15, 16, 17 Other early applications included assessing the nature and extent of air pathway concerns related to the petroleum industry 18,19 as well as chemical wastewater treatment facilities 20 and municipal wastewater treatment facilities including odor emissions. 21,22,23 The technology has been widely used for assisting in site assessment programs that required an investigation of the air pathway for future land redevelopment 24, 25,26 and numerous other applications including land treatment, landfill disposal, agricultural, and industrial applications are documented. 27 However, the first application for assessing infiltration through seam and cracks came with the need to assess petroleum emissions through cement slab seams in an oil/water separator unit at a petroleum refinery. 28 Assessing impact on open soil over groundwater plumes, for instance, was a straightforward application of the technology 29, 30 however, the combination of outdoor, open soil flux testing over a known subsurface source (typically a dissolved-phase groundwater plume) with infiltration through a slab provided for adequate assessment of the vapor infiltration into structures. 31-36 Testing at sites where predictive modeling was conducted as described using proper site characterization data showed the conservative nature of the J&E model and use of the screening tool as a conservative estimate of potential exposure. 37, 38, 39 As such, the 'second step' using direct measured flux data, provides for the assessment of vapor infiltration.

Air Pathway Analysis Using Predictive Modeling, the USEPA Flux Chamber and Ambient Air Monitoring

For most sites, the second step after predictive modeling has indicated that the potential for vapor infiltration exists involves a combination of both recommended direct measurement technologies: flux chamber assessment using the USEPA recommended flux chamber, and ambient air monitoring indoor and outdoor ambient air. The concept is to study the source of potential exposure, the pathway of vapor intrusion, and the affect of vapor infiltration into the subject structure. A balanced air pathway analysis might include an adequate assessment of open soil, outdoor flux testing over the known subsurface source and near/around the structure foundation. This data set (open soil, outdoor flux) is intended to define the extent of impact at the surface, typical and maximum flux potential to the structure slab or sub-floor, and the full range of chemical species that is probably involved in the infiltration into the structure. An estimate of potential indoor infiltration can be obtained by using the outdoor flux data and an assumed infiltration or crack fraction to estimate indoor, endpoint concentration. The conservative ASTM crack fraction is 1% of the slab surface area, but other estimates based on civil engineering analysis reported in the literature is 0.056%. 40 As such, this activity provides for a source assessment of the area source as well as an estimate of potential infiltration and indoor exposure. It is important to note that for most studies, background flux in the urban environment is significant and background flux data should be collected representative of the local air shed but not related to the subsurface contamination on the study site. Measurable levels of ubiquitous volatile organic compounds are routinely measured at the sub-ppbv level in the flux chamber (above method and system blank levels) that background levels need to be considered when assigning surface flux to a known subsurface source. 41 This is especially important for a common air contaminant such as benzene and other components found in petroleum fuels, urban air, and in contaminated groundwater from surface and subsurface releases to the environment.

Indoor infiltration testing using the flux chamber typically involves testing on uncovered cement slab or sub-flooring at likely points of infiltration. Both uncracked or continuous slab and seamed/cracked slab are tested. Screening is typically performed using a field portable instrument or an onsite laboratory capability and locations for flux chamber testing are selected and tested. The flux chamber sealing on the hard slab is recommended and adequate testing is required to assess mass transfer from seams and cracks in the slab. If uncovered slab surfaces are not available, testing is typically conducted on sidewalks or patios around the structure. Data are generated as mass emitted per linear feet of seam/crack, prorated to the footprint of the surface area of the slab that is affected, and infiltration flux calculated is input into a ventilation calculation resulting in an endpoint exposure concentration (indoor) for health risk assessment. These data provide a second and more representative estimate of exposure potential and represent the "pathway" of migration as opposed to the "source" of infiltration as represented by the outdoor, open soil flux. Note that the USEPA flux chamber measures flux at the ambient pressure the measurement is performed (vented chamber). As such, the building pressure, negative or positive in comparison to outdoor ambient pressure is conveyed in the measurement. Most structures are at positive or ambient pressure, however, this variable is accounted for in the proper use of the USEPA recommended flux chamber design and operation. Data can be colleted at induced pressures (e.g., -4Pa) if indoor underpressurization or negative pressure compared to ambient pressure affecting infiltration flux is a project concern. 42

The third component of the air pathway analysis assessing vapor infiltration is a limited amount of indoor and outdoor ambient air testing. These data, though not intended to serve as input to a health risk assessment, can be used to verify the results of the open soil outdoor flux testing and indoor infiltration flux assessment of indoor exposure. The concern is that significant points of infiltration into the structure might be overlooked in the indoor infiltration survey resulting in a low estimate of infiltration. A screening-level data set of indoor/outdoor air, typically collected during the direct flux chamber assessment, provides for a verification of testing protocol and endpoint comparison data (calculated endpoint indoor concentration versus screening-level measured ambient indoor concentration).

Summary

The Draft Guidance provides for an air pathway analysis approach applied to the unique exposure scenario of infiltration of vapors from subsurface sources. The Guidance, however, does not provide adequate direction for those sites that demonstrate an exceedence of health criteria or have the potential for exposure to occupants/residents via infiltration into structures. Comments have been provided that include a discussion of technically sound and referenced testing methodology that are reported to generate representative, 'in-depth' level site assessment data that can be used as input to health risk assessment. An air pathway analysis, consisting of predictive modeling using site specific data and robust ambient air monitoring, or predictive modeling using site specific data and outdoor and indoor flux chamber testing using the USEPA surface emission isolation flux chamber coupled with screening-level indoor and outdoor ambient air monitoring, constitute viable approaches for assessing the exposure potential of vapor infiltration from subsurface sources.

REFERENCES

1. Johnson, P.C. and R.A. Ettinger, Environmental Science and Technology , Vol 25, 1445-1452, 1991.

2. USEPA 1990. Air/Superfund National Technical Guidance Study Series , "Volume 2- Estimation of Baseline Air Emissions at Superfund Sites," EPA-450/1-89-002a, August 1990.

3. USEPA 1992. Engineering Bulletin: Air Pathway Analysis , EPA/540/S-92/013, November 1992.

4. Massachusetts Department of Environmental Protection (MA DEP) Indoor Air Sampling and Evaluation Guide- WSC Policy #02-430 , April 2002.

5. USEPA 1992. Engineering Bulletin: Design Considerations for Ambient Air Monitoring At Superfund Sites , EPA/540/S-92-012, November 1992.

6. Hers, Ian, et .al., "The Use of Indoor Air Measurements To Evaluate Intrusion of Subsurface VOC Vapors into Buildings." JAWMA , Volume 51, September 2001

7. USEPA 1986. Measurement of Gaseous Emission Rates from Land Surfaces Using an Emission Isolation Flux Chamber, User's Guide , EPA Contract No. 68-02-3889, WA #18, February 1986.

8. Schmidt, C.E., W.D. Balfour, and R.D. Cox. "Sampling Techniques for Emission Measurements at Hazardous Waste Sites," Proceedings of the 3rd National Conference and Exhibition on Management of Uncontrolled Hazardous Waste Sites, Washington, D.C., November 1982.

9. Schmidt, C.E. and W.D. Balfour. "Direct Gas Emission Measurement Techniques and the Utilization of Emissions Data from Hazardous Waste Sites," Presented at the 12th Annual Conference on Waste Technology, Memphis, TN, October 1983.

10. Schmidt, C.E., W.D. Balfour, and R.D. Cox. "A Direct Emission Sampling Tech­nique for Measurement of Toxic Gaseous Contaminants," Presented at the National Conference on Environmental Engineer, Boulder, CO, July 1983.

11. Eklund, B.M., W.D. Balfour, C.E. Schmidt. "Measurement of Fugitive Volatile Organic Compound Emission Rates with an Emission Isolation Flux Chamber," Proceedings of the AICHE 1984 Summer National Meeting, Philadelphia, PA, August 1984.

12. Eklund, Bart, "Practical Guidance for Flux Chamber Measurements of Fugitive Volatile Organic Emission Rates," JAWMA , Vol 42, December 1992, pp1583-1591.

13. Eklund, B.M., W.D. Balfour, C.E. Schmidt. "Measurement of Fugitive Volatile Organic Emission Rates," Environmental Progress, August 1985, pp 199-202.

14. Clark, J.A., C.E. Schmidt, T. D'Auanzo, "Overview of Applicable Emission Measurement Technologies for the Measurement of Volatile Hazardous Waste Emissions, "Proceedings of the 1988 EPA/APCA Symposium on Measurement of Toxic and Related Air Pollutants, Raleigh, North Carolina, May 1988.

15. Schmidt, C.E. and M.W. Eltgroth. "Off-Site Assessment of Air Emissions from Hazardous Waste Disposal Facilities," Proceedings of the 4th National Conference and Exhibition on Management of Uncontrolled Waste Sites, Washington D.C., November 1983.

16. Millison, Dan, Barbara Marcotte, Caroline Rudolph, Karen Randles, DTSC/CAL EPA, "Applications and Comparisons of Soil Gas, Flux Chamber, and Ambient Air Sampling Risk Assessment at a Hazardous Waste Site," HMC , July/August 1991.

17. McDonough, M., C.E. Schmidt. "Procedures for Conducting Air Pathway Analyses at Superfund Sites - Baseline Emission Estimates," 82nd Annual Meeting of the Air and Waste Management Association, Anaheim, California, June 1989.

18. Vandervort, R., C.E. Schmidt, W.D. Balfour. "Surface and Subsurface Gas/Vapor Monitoring Techniques Applied to Environmental Contamination Caused by Petro­leum Products and Processing Waste," Proceedings of American Petroleum Insti­tute Committee Meeting on Safety and Fire Protection, San Antonio, TX, September 1984.

19. Schmidt, C.E., R. Vandervort, W.D. Balfour, "Technical Approach and Sampling Techniques Used to Detect and Map Subsurface Hydrocarbon Contamination," 86-75.4, 79th Annual Meeting of The Air Pollution Control Association, San Francisco, California, June 1986.

20. Nottoli, J., C.E. Schmidt, R. Boyd, "Measurement of Toxic Air Contaminants From an Industrial Wastewater Treatment Plant", 92-99.08, 85th Annual Meeting of the Air and Waste Management Association, Kansas City, Kansas, June 1992.

21. Shen, T., C.E. Schmidt, T. Card, Assessment and Control of VOC Emissions From Waste Treatment and Disposal Facilities , Van Nostrand Reinhold Publishing, New York, New York, 1993.

22. Schmidt, C.E., William Faught, Judy Nottoli, "Using the EPA Recommended Surface Emission Isolation Flux Chamber to Assess Emissions from Aerated and Non-Aerated Liquid Surfaces," Proceedings of the 1991 EPA/AWMA Symposium on Measurement of Toxic and Related Air Pollutants, Raleigh, North Carolina, May 1991.

23. Schmidt, C.E., et.al., "Assessment of Odor Emissions Using the US EPA Flux Chamber and Olfactory Odor Measurement," ­­­96-FA147.04, 89th Annual Meeting of the Air and Waste Management Associa­tion, Nashville, TN, June, 1996.

24. Schmidt, C.E., M., Simon, "Application of a Direct Emission Assessment Technology for Conducting Site Investigations for Subsurface Contamination", 93-MP-22.04, 86th Annual Meeting of the Air and Waste Management Association, Denver, Colorado June 1993.

25. Schmidt, C.E., et.al., "Air Pathway Analysis in Support of Remediation of the Rocky Mountain Arsenal," ­­­96-TP40.05, 89th Annual Meeting of the Air and Waste Management Associa­tion, Nashville, TN, June, 1996.

26. Thalheimer, Andrew, Mark Jones, Schmidt, C.E., "Measured Flux in an Air Pathway Analysis as a Necessary Step in Site Remediation," Paper No. 76, Air and Waste Management Association Symposium on Air Quality Measurement Methods and Technology, San Francisco, California, November 13-15, 2002.

27. Schmidt, C.E., "Theory and Application of the EPA Recommended Surface Emission Isolation Flux Chamber- Assessment of Volatile and Semi-Volatile Species Emission Rates," Chapter 3, Sampling and Analysis of Airborne Pollutants , Eric Winegar and Larry Keith editors, Lewis Publishers, Ann Arbor, Michigan, 1993.

28. Schmidt, C.E., J.A. Clark, "Use of the Surface Isolation Flux Chamber to Assess Fugitive Emissions From a Fixed-Roof on an Oil-Water Separator Facility." Proceedings of the 1990 EPA/AWMA Symposium on Measurement of Toxic and Related Air Pollutants, Raleigh, North Carolina, May, 1990.

29. Schmidt, C.E., A.S. Johnson, "Technical Approach Developed to Assess the Volatilization and Migration of Volatile Organic Compounds From Contaminated Ground Water." Proceedings of the 1989 EPA/AWMA Symposium on Measurement of Toxic and Related Air Pollutants, Raleigh, North Carolina, May 1989.

30. Bejorklund, Brian, Schmidt, C.E., Robin Streeter, "Air Pathway Analysis Characterizing Potential Exposure from a Dissolved-Phase Groundwater Plume using Direct Flux Measurement," Paper No. 64, Air and Waste Management Association Symposium on Air Quality Measurement Methods and Technology, San Francisco, California, November 13-15, 2002.

31. Schmidt, C.E., T.F. Zdeb, "Direct Measurement of Indoor Infiltration Using the US EPA Flux Chamber and Dispersion Modeling", 98-TA9C.01, 91th Annual Meeting of the Air and Waste Management Associa­tion, San Diego, California, June, 1998.

32. Schmidt, C.E., Jared Rubin, A Indoor Infiltration Assessments of VOCs from Contaminated Groundwater Using the US EPA Flux Chamber @ , Paper No. 446, 93 rd Annual Meeting of the Air and Waste Management Association, Salt Lake City, Utah, June, 2000.

33. Copeland, Teri, C.E. Schmidt, Jim Van de Water, Michael Manning, "Predicting Potential Exposure fro the Occupants in Future Buildings Using Direct Measurement and Predictive Modeling Techniques," Paper No. 43141, 95 th Annual Meeting of the Air and Waste Management Association, Baltimore, MD, June, 2002.

34. Boehnker, David, John Tiffany, Schmidt, C.E., "Estimating Exposure to Residents With Basements Using Measured Surface and Subsurface Flux Data," Paper No. 67, Air and Waste Management Association Symposium on Air Quality Measurement Methods and Technology, San Francisco, California, November 13-15, 2002.

35. Richter, Rich, Schmidt, C.E., "Assessing Realistic Risk to Indoor Occupants from Subsurface VOC Contamination," Paper No. 69, Air and Waste Management Association Symposium on Air Quality Measurement Methods and Technology, San Francisco, California, November 13-15, 2002.

36. Robles, Heriberto, C.E. Schmidt, Teri Copeland, Jim Van de Water, Michael Manning, "Measured Infiltration Flux as the Preferred Input to Indoor Exposure Assessment," Paper No. 70, Air and Waste Management Association Symposium on Air Quality Measurement Methods and Technology, San Francisco, California, November 13-15, 2002.

37. Schmidt, C.E., Teri Copeland, Rich Pesin, "Comparison of Measured and Modeled Emissions from Subsurface Contamination at an Industrial Site in a Residential Neighborhood", 98-WPC.01, 91th Annual Meeting of the Air and Waste Management Association, San Diego, California, June, 1998.

38. Schmidt, C.E., et.al., "Comparison of Measured Versus Modeled Surface Flux of VOCs from Contaminated Groundwater @ , Paper No. 447, 93 rd Annual Meeting of the Air and Waste Management Association, Salt Lake City, Utah, June, 2000.

39. Menatti, John, Ed Fall, "Measured Versus Modeled Exposure from Petroleum Groundwater Plumes," Paper No. 74, Air and Waste Management Association Symposium on Air Quality Measurement Methods and Technology, San Francisco, California, November 13-15, 2002.

40. Sager, Shawn, Laura Braddy, Christopher Day, "The Infiltration Ratio in Vapor Infiltration Intrusion Calculations," Presented at the Society for Risk Analysis Annual Meeting, Washington D.C., December 9, 1997.

41. DeVaull, G.E., J.B. Gustafson, C.E. Schmidt, "Surface Emission Flux Measurements: Back­ground Levels," Proceed­ings of the Fourth Annual West Coast Regional Specialty Confer­ence on Current Air Toxic Issues, AWMA, November, 1994.

42. Sheldon, Andrew, and C.E. Schmidt, "Evaluation of an Underpressurized Emission Flux Chamber for Measuring Potential Subsurface Vapor Intrusion Into Buildings," Paper No. 42690, 95 th Annual Meeting of the Air and Waste Management Association, Baltimore, MD, June, 2002.