Predicting Potential Exposure for Occupants of Future Buildings Using Direct Measurement and Predictive Modeling Techniques

摘要

The redevelopment of industrial properties, or properties adjacent to decommissioned industrial properties, poses challenges for the health risk assessor. Often, a health risk assessment is required as a component of the redevelopment of a site that has, or is, undergoing some level of soil and/or groundwater remediation. Typically, the redeveloped site has to contend with low-level, residual contamination that may result in air pathway exposure for future occupants of the site. One accepted technology used to assess this potential exposure is use of a predictive model such as the Johnson and Ettinger (J&E) model or related models referenced in USEPA guidance. These models rely on groundwater, soil, or soil gas data to predict indoor air concentrations in future structures. The J&E and related models are 'discovery' in nature and conservative by design, and are, therefore, commonly employed as the basis of conservative screening risk assessments in cases where the conceptual site model is consistent with the assumptions of the model. Risk assessments that conclude no significant health risk based on the conservative predictive modeling approach may be used as the basis for proceeding with redevelopment. Risk assessments that indicate potential for risk, based on the results of a conservative predictive model, provide the rationale for further site characterization and/or a higher tier risk assessment. The purpose of this paper is to discuss what is considered by the USEPA as an appropriate 'next step' for conducting site assessment for those sites requiring a more detailed or 'in-depth' site assessment and exposure assessment for the air pathway. The USEPA describes this procedure in the Technical Guidance Series: Procedures for Conducting Air Pathway Analyses for Superfund Sites. Although not specific to Superfund sites, this technical guidance series describes technologies for collecting data for refined exposure assessment. One such technology used for this purpose is the USEPA surface emission isolation flux chamber. This technology can provide direct measurement data that can be employed in the air pathway exposure evaluation to reduce uncertainties inherent in predictive modeling approaches. The technology involves conducting emission measurements on open soil (e.g., a future building footprint), assuming non-advective flow conditions, and estimating a credible, reasonable maximum infiltration factor for soil gas. The key components of the methodology that will be discussed in the paper include the identification of data quality objectives, flux chamber testing to determine diffusion-based flux at the land surface on open soil, and application of flux chamber data in exposure assessment. Site data from a project where both predictive modeling and direct measurement were used in the health risk assessment are presented and used to support the direct measurement approach. Comparisons are used to demonstrate the conservative nature of the screening approach that uses predictive modeling and to support uncertainty analysis of HRA components (e.g., identification of chemicals of potential concern and dose estimates).