Flux Chamber Measurements:Defensible Analytical Data for Evaluating Human Health RiskPosition Paper Written for San Diego Department of Environmental Health Submitted by Anne M. Babyak, Principal Scientist, BBL Sciences, on behalf of Atlantic Richfield Company, ARCO Facility No. 5132, San Diego , California Introduction While ATSM recommends soil gas as an ideal reconnaissance tool and screening technique, soil gas monitoring does not provide repeatable quantitative information over time due primarily to the dynamic nature of phase equilibria in the vadose zone and secondarily to unavoidable inconsistencies in sampling practice. And, therefore, soil gas data is at best a good screening tool for human health risk assessment but it does not provide an accurate and consistent measure of exposure for risk assessment. The EPA recommended flux chamber is an excellent alternative to indoor/outdoor air sampling to verify predicted indoor air results from screening level models. It has been developed and used over the past 20 years and EPA continues to recommend its use through their guidance documents. The methodology is tested and documented. A number of scientists have published its use in verifying the accuracy of model predictions. The data from flux chamber samplers is quantitative and representative for use in human health risk assessment. Limitations on Soil Gas Screening Local, State and Federal agencies, including the San Diego Department of Environmental Health Site Assessment and Mitigation Division (SAM), use screening tools to determine whether a site requires further evaluation and remediation. Some screening tools are analytical models which are commonly used to determine if volatile contaminants in subsurface soil and groundwater may be migrating into buildings and posing adverse health risks to people. For example, SAM uses Vapor Risk 2000 as an analytical model to estimate indoor air risk from subsurface sources. These analytical models, which are built on a series of equations, are conservative due to uncertainties in modeling volatile contaminants partitioning from subsurface soil and groundwater, diffusing through the vadose zone, and migrating through concrete foundations into building air. The environmental data commonly input into these equations are soil, groundwater or soil gas data. If soil or groundwater data are used, then the model must account for partitioning of contaminants through the soil to the building foundation, depth of source, soil permeability, and depletion of source. EPA and other agencies have recommended the use of soil gas surveys in conjunction with these models in order to eliminate the need to model partitioning of contaminants, migration from source to the building foundation, and soil permeability (EPA 1996). Soil gas data has its limitations, however, because soil gas data is strongly dependent upon effects related to geologic variation and moisture content in the sampling horizon. ASTM (1995a) says that soil gas monitoring is a method to suggest the presence, composition, and origin of contaminants, but the results reflect the interaction of partitioning, migration, emplacement and degradation of soil gas in a dynamic not static equilibrium. For this reason, soil gas datasets based on specific contaminant concentrations and generated at different times are usually not comparable. The act of sampling itself disrupts the equilibrium conditions in the subsurface. The internal volume of the ground probe can significantly affect the measurement process and the utility of the resulting data. While ATSM recommends soil gas as an ideal reconnaissance tool and screening technique, soil gas monitoring does not provide repeatable quantitative information over time due primarily to the dynamic nature of phase equilibria in the vadose zone and secondarily to unavoidable inconsistencies in sampling practice. For these reasons, ASTM asserts that soil gas data in itself cannot be used to provide definitive answers about the location or absence of buried contaminants (ASTM 1995a). Human health risk assessments rely on usable quantitative data (EPA 1992). Data need to be representative of exposure point concentrations through time because risk assessment estimate exposure over long periods of time, up to 30 years for residents and 25 years for commercial/industrial situations. Risk assessments also need to be based on data that are of defensible analytical quality (EPA 1992). Therefore, risk assessments based on soil, groundwater or soil gas data input into equations to estimate indoor air are perfectly acceptable as a screening level evaluation. However, a site that fails a screening level assessment may warrant further evaluation. For this reason, EPA, ASTM and state agencies recommend a tiered approach to risk evaluation (EPA 1986, ASTM 1995b, 2000). There are sampling methods designed to validate the results of indoor air models which provide more suitable data for quantitative risk evaluations. Determining the emission rates of volatile organic compounds from soil and groundwater has been the subject of much investigation over the last 18 years and the emission isolation flux chamber has emerged through EPA guidance and scientific literature as the primary method to gather this information. What is a Flux Chamber? Gaseous emissions are collected from an isolated surface area with an enclosure device called an emission isolation flux chamber. Clean, dry sweep air is added to the chamber at a fixed, controlled rate (e.g., 0.005 m 3 /min) that is selected based on site conditions. The volumetric flow rate of sweep air through the chamber is recorded and the concentration of the constituent of interest is measured at the exit of the chamber. The emission rate is calculated based upon the surface area isolated, the sweep air flow rate, and the gaseous concentration measured. An estimated average emission rate for the area source is calculated based upon statistical sampling of a defined total area. The development of this method started in the early 1980s when EPA needed more information on measuring emission rates from hazardous waste sites. How was the Flux Chamber Methodology Developed? In 1985, EPA summarized three general approaches for determining gas emission rates from land surfaces contaminated with VOCs: indirect measurements, direct measurements, and laboratory simulations (EPA 1985). The literature and the three approaches were compared for precision, accuracy and sensitivity. EPA concluded that the most promising technique for measuring gas emission rates from land surfaces was the emission isolation flux chamber. The advantages were summarized by EPA as:
In 1990, EPA published Volume II of the National Air Emissions Series: Estimation of Baseline Air Emissions at Superfund Sites (EPA 1990). EPA cited the flux chamber as the preferred technology for measuring subsurface emissions rates and indicated that the advantages generally out-weigh the limitations of the technique. Eklund (1992) published a paper on practical guidance for flux chamber measurements. The author cited four reasons for publishing the guidance after EPA had already published a User's Guide in 1986: (1) copies of the flux chamber user's guide and development studies are not readily available, (2) the flux chamber design is not readily available from vendors, (3) the recommended design is difficult to fabricate so users opt for simpler but unvalidated models, and (4) the performance characteristics are dependent on the design so substituted designs may not give good quality control. In 1993, Winegar and Keith published Sampling and Analysis of Airborne Pollutants in which Chapter 3 gives an excellent summary of the theory and application of the EPA's recommended surface emission flux chamber. In 1996, EPA used the flux chamber model to validate the Jury Infinite Source model and presented their findings in the Soil Screening Level Guidance Technical Background Document (EPA 1996). In Appendix C of the Soil Screening Level guidance, EPA summarizes why flux chambers studies were chosen to provide pilot scale or field-scale measurement data needed for model validation. The guidance says, "Flux chambers have been widely used to measure flux rates of VOCs and inorganic gaseous pollutants from a wide variety of sources. The flux chamber was originally developed by soil scientists to measure biogenic emissions of inorganic gases and their use dates back at least two decades (Hill et al., 1978). In the early 1980s, EPA became interested in this technique for estimating emission rates from hazardous wastes and funded a series of projects to develop and evaluate the flux chamber method. The initial work involved the development of a design and approach for measuring flux rates from land surfaces. A test cell was constructed and parametric tests performed to assess chamber design and operation (Kienbusch and Ranum, 1986 and Kienbusch et al. 1986). A series of field tests were performed to evaluate the method under field conditions (Radian Corporation 1984 and Balfour et al., 1984). A User's Guide was subsequently prepared summarizing guidance on the design, construction, and operation of the EPA recommended flux chamber (Keinbusch 1985). The emission isolation flux chamber is presently considered the preferred in-depth direct measurement technique for emissions of VOCs from land surfaces (EPA 1990)." Since that time, there have been a number of articles documenting the use of surface flux chambers, mostly to validate the use of the Johnson and Ettinger model or other analytical models to estimate indoor air concentrations. Examples of the Use of Flux Chambers Table 1 provides a dozen sites (most located in Southern California ) which have gained regulatory approval for the use of flux chamber sampling. This list is hardly exhaustive but gives an idea of the situations in which regulatory agencies have used or approved the use of flux chamber data. Also, a paper by Eklund et al. (1991) which summarizes emission rate measurement projects is available through EPA's National Technical Information System (NTIS). Millison et al. (1991) published a case study in Southern California in which soil gas, flux chamber and ambient air results were used. The authors (including three representatives of CalEPA Department of Toxic Substances Control) concluded that flux chamber and ambient air sampling provided the data necessary for estimating inhalation exposure at homes in the residential neighborhood. Menatti and Fall (2002) (representing Utah Division of Environmental Response and Remediation (DERR)) presented two cases studies in Utah where benzene flux rates were estimated using ATSM RBCA, DERR RBCA and San Diego County SAM Manual. These flux rates were compared to data collected by the emission isolation flux chamber recommended by EPA. The models overpredicted flux in both case studies and the authors concluded that the models do not account for biodegradation and other factors. There are four papers documenting one field validation of the soil gas to indoor air pathway in which flux chamber results were compared to three different models including the Johnson and Ettinger model (Hers et al., 2003, 2002, 2001, and Hers and Zapf-Gilje 1998). The papers document the overprediction by models except in the case of building underpressurization (advection) case which is primarily caused by pressure differentials that may occur during winter with residential heating. The authors noted that soil moisture is a critical parameter that influences all the major processes affecting vadose zone fate and transport (i.e., diffusion, advection, and biodegradation) (Hers and Zapf-Gilje 1998). Numerous factors potentially affect the vapor intrusion pathway including biodegration, chemical transformation, sorption, contaminant source depletion, geologic heterogeneity, soil properties (moisture content, permeability, organic carbon content), building properties, meterological conditions, and building ventilation rates (Hers et al., 2003). How Good are the Data? EPA (1985) and Eklund (1992) summarize the quality of the flux chamber methodology. The single chamber repeatability is approximately 5 percent at measured emission rates of 3,200 ug/m 2 -min. Variability between flux chambers (i.e., reproducibility) is approximately 9.5 percent within a measured emission rate range of 39,000 to 65,000 ug/m 2 -min. Flux chamber recovery results show a recovery range of 77 to 124 percent. The average recovery of 40 compounds tested was 103 percent. Flux chamber emission rate measurements made on the soil cells range from 50 to 100 percent of the predicted emission rates and, therefore, can be expected to be within a factor of one-half of the actual emission rate. The flux chamber accuracy based upon both the recovery tests and predictive modeling ranges from 50 to 124 percent. The sensitivity of the method depends of the detection limit of the analytical technique used. When discrete samples are collected using gas canisters and analyzed by gas chromatographic methods, the estimated emission rate sensitivity is0.0012 ug/m 2 -min for an analytical detection limit of 0.010 ppbv benzene. Summary The EPA recommended flux chamber is an excellent alternative to indoor/outdoor air sampling to verify predicted indoor air results from screening level models. It has been developed and used over the past 20 years and EPA continues to recommend its use through their guidance documents. The methodology is tested and documented. A number of scientists have published its use in verifying the accuracy of model predictions. The data from flux chamber samplers is quantitative and representative for use in human health risk assessment. References American Society for Testing and Materials (ASTM, 1995a). Standard Guide for Soil Gas Monitoring in the Vadose Zone. D 5314-92 (Reapproved 2001). West Conshohocken , Pennsylvania . ASTM. 1995b. Risk-Based Corrective Action Applied at Petroleum Release Sites. E 1739-95. West Conshohocken , Pennsylvania . ASTM. 2000. Standard Guide for Risk-Based Corrective Action. E 2081-00. West Conshohocken , Pennsylvania . Balfour, W.D., Eklund, B.M. and Williamson, S.J. 1984. Measurement of Volatile Organic Emissions from Subsurface Contaminants. In Proceedings of the National Conference on Management of Uncontrolled Hazardous Waste Sites (pp. 77-81). Hazardous Materials Control Research Institute, Silver Springs , Maryland . September. Eklund, B., Petrinec, C., Ranum, D. and Howlett, L. 1991. Database of Emission Rate Measurement Projects - Draft Technical Report. EPA/450/1-91/003 (NTIS No. PB222059LDL). May. Eklund, B. 1992. Practical Guidance for Flux Chamber Measurements of Fugitive Volatile Organic Emission Rates. J. Air Waste Mange Assoc 42:1583-1591. Hers, I, Zapf-Gilje, R., Li, L. and Atwater , J., 2001. The Use of Indoor Air Measurements to Evaluate Intrusion of Subsurface VOC Vapors into Buildings. J. Air & Waste Manage. Assoc. 51:1318-1331. Hers, I. , Zapf-Gilje, R., Evans, D., and Li, L. 2002. Comparison, Validation and Use of Models for Predicting Indoor Air Quality from Soil and Groundwater Contamination. J. of Soil and Sediment Contamination. 11(4):491-527. Hers, I, Zapf-Gilje, R., Johnson, P. and Li, L. 2003. Evaluation of the Johnson and Ettinger Model for Prediction of Indoor Air Quality. Groundwater Monitoring and Remediation . 23(1):62-76. Johnson, P.C. and Ettinger, R.A. 1991. Heuristic Model for Predicting the Intrusion Rate of Contaminated Vapors into Buildings. Environmental Science and Technology. 25:1445-1452. Menatti, J. and Fall, E. 2002. Soil Vapor: The Phantom Menace? A Comparison of Surface Emission Flux Chamber Measurements to Modeled Emissions from Subsurface Contamination. 95 th Annual Meeting of the Air and Waste Management Association, Baltimore , MD. Paper No. 42734, June. Millison, D., Marcotte, B., Rudolph, C. and Randles, K. 1991. Applications and Comparison of Soil Gas, Flux Chamber and Ambient Air Sampling Results to Support Risk Assessment at a Hazardous Waste Site . Hazard Management Consultant , July/August. Hill, F.B., Aneja, V.P., and Felder, R.M. 1978. A Technique for Measurement of Biogenic Sulfur Emission Fluxes . J. Env. Sci. Health AIB (3):199-225. Kienbusch, M. 1985. Measurement of Gaseous Emission Rates from Land Surfaces Using an Emission Isolation Flux Chamber - User's Guide. Report to EPA-EMSL, Las Vegas under EPA Contract No. 68-02-3889. Work Assignment 18, December 1985. Kienbusch, M. and Ranum, D. 1986. Validation of Flux Chamber Emission Measurements on a Soil Surface. Draft Report to EPA-EMSL, Las Vegas , Nevada . Kienbusch, M, Balfour, W.D., and Williamson, S. 1986. The Development of an Operations Protocol for Emission Isolation Flux Chamber Measurements on Soil Surfaces. Presented at the 79 th Annual Meeting of the Air Pollution Control Association (Paper No. 86-20.1), Minneapolis , Minnesota , June 22-27. Radian Corporation. 1984. Soil Gas Sampling Techniques of Chemicals for Exposure Assessment - Data Volume. Report to EPA-EMSL, Las Vegas under EPA Contract No. 68-02-3513, Work Assignment 32, March. United States Environmental Protection Agency (USEPA). 1989. Risk Assessment Guidance for Superfund , Volume I, Human Health Evaluation Manual (Part A). Office of Emergency and Remedial Response, December. USEPA. 1985. Measurement of Gaseous Emission Rates From Land Surfaces Using an Emission Isolation Flux Chamber, Users Guide . EPA Environmental Monitoring Systems Laboratory, Las Vegas , Nevada . NTIS No. PB-86-223161. December. USEPA. 1990. Procedures for Conducting Air Pathway Analyses for Superfund Activities, Interim Final Documents: Volume 2 - Estimation of Baseline Air Emissions at Superfund Sites. Office of Air Quality Planning and Standards. EPA/450/1-89/002a. July. USEPA. 1996. Soil Screening Guidance: Technical Background Document. Office of Solid Waste and Emergency Response, May. USEPA. 1992. Guidance for Data Usability in Risk Assessment (Part A), Final. Office of Emergency and Remedial Response, April. Winegar, E.D. and Keith, L.H. 1993. Sampling and Analysis of Airborne Pollutants , C.E. Schmidt, Chapter 3, "Theory and Applications of the USEPA Recommended Surface Emission Isolation Flux Chamber for Measuring Emission Rates of Volatile and Semivolatile Species". Lewis Publishers, Ann Arbor , Michigan . The reproducibility results were determined from a bench-scale study. The tests were designed to eliminate temporal variations from the flux chamber reproducibility. However, using the same bench-scale facility, a test design was not possible for measuring flux chamber repeatability without bias from temporal variations. As a result, the repeatability tests were performed in the laboratory. The differences therefore between the stated emission rates for repeatability and reproducibility reflect the differences in laboratory simulated emission rates and those measured from the bench-scale facility.
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