'Evaluating Direct Measurement Approaches Used forAssessing Potential Air Pathway Impacts to Occupants inStructures Over Subsurface Sources'

摘要

Impacts from subsurface sources, including groundwater plumes, contaminated soils, andmigrating soil gas from various nearby sources presents the potential for exposure to occupantsin structures. Site assessment for vapor intrusion (VI) into structures as indicated by the USEPAincludes using a 'tiered' assessment approach where typically the presence of possible soil gassources are evaluated by studying site records (Tier 1), then if necessary, the contaminationsource below the structure is studied (e.g. groundwater collection and analysis, soil sampling andanalysis, or soil gas sampling and analysis) and a 'source term' is used in a predictive model toestimate the potential exposure (Tier 2).1 This approach, assuming that the predictive modeling(Johnson & Ettinger equation2 or other predictive equations) describes the site conditionsaccurately and the source term data are representative, generally results in a conservativeassessment of potential exposure to occupants in the subject structure. If the assessment resultsin an estimate of health risk that is acceptable (e.g., increased cancer risk of less than one in amillion and a hazard index of less than one), then the pathway is considered 'incomplete' and nofurther assessment is recommended. But if the Tier 2 assessment results in an estimate of healthrisk that is above criteria, then further assessment (Tier 3) is recommended in order to generate amore refined or representative estimate of potential exposure and health risk to occupants.Additional information including more source data, can be collected and the predictive modelingenhanced, or direct measurement technologies can be used to provide parallel lines of evidencethat can be used to demonstrate the intrusion potential, or support a refined health riskassessment. There are two main technical approaches available for assessing potential exposure to occupantsin structures that are applicable for Tier 3 level assessment other than subslab soil gas testing andrefined predictive modeling, including: 1) the direct measurement of flux using the USEPA surface emission isolation flux chamber; and 2) the measurement of indoor and outdoor ambientair quality in the subject structure. The application of these direct measurement technologiesto the 'vapor intrusion site' will be presented along with a discussion of the advantages andlimitations of these technologies. The goal of the paper is to provide information that is useful in assessing the air pathway for vapor intrusion using in-depth, direct measurement technologies.Case study information will be provided demonstrating the comparison of exposure assessmentfrom both technologies conducted simultaneously at a case study site. Flux chamber andindoor/outdoor ambient air testing technologies are presented as complimentary technologiesthat, when used in a comprehensive VI design provides for a more complete understanding ofpotential exposure from the subsurface source.