Theme: 09. Vapor Intrusion

Topic Area: 9a. Advances in Vapor Intrusion Investigations

Formatted Title:

Empirical Building-Specific Attenuation Factor Determination via Automated Continuous Monitoring of Chemical and Physical Parameters

Background/Objectives:

The vapor intrusion (VI) attenuation factor (AF) represents a key risk decision criterion for regulators, commercial entities, consultants, and developers. In simplest terms, the AF is the ratio between an indoor air contaminant concentration and the concentration observed in the contaminated media (e.g., soil gas or groundwater). A default screening AF is commonly used to decide whether to sample indoors or to implement mitigation. In general, practitioners perform a soil vapor survey or groundwater sampling campaign prior to sampling indoors. Key determinants regarding whether to sample indoors include the anticipated indoor concentration and whether this predicted concentration exceeds a risk screening threshold established by the local or regional regulatory agency. Many federal and state regulatory agencies set 0.03 as the default screening AF for the soil gas-to-indoor vapor pathway. Others have argued that this value is too conservative. To date, every study has relied upon paired indoor air and subsurface sample results using randomly-timed time-integrated indoor air samples. Researchers have shown how sample timing can have a profound impact on indoor air results. More specifically, sampling when VI is “on” (e.g., during upward vapor flux conditions) may represent a superior estimate of an indoor air concentration that adheres to USEPA’s recommended risk decision criterion described as the “reasonable maximum exposure” (RME). Project objectives include the following:

document differences that may exist between continuous high-frequency monitoring and 24-hr time weighted average vapor samples;
improve our understanding of VI dynamics (e.g., relationships between indoor air concentration, concurrent subsurface concentration, differential pressure, climate, etc.) via high-frequency monitoring;
determine whether vapor sampling when VI is “on” (e.g., upward flux conditions) provides a representative risk screening approach and assessment of empirical building-specific AF criteria.

Approach/Activities:

An investigation was performed at two shopping centers with former drycleaner releases of tetrachloroethylene (PCE) to help resolve the ongoing debate and determine a range of empirical AF values that account for indoor air concentration variability. More specifically, high-frequency automated continuous monitoring of chemical and physical parameters was performed over multiple days to accomplish the project objectives. The automated continuous monitoring system is comprised of a customized laboratory-grade gas chromatograph equipped with various detectors for rapidly measuring TCE, PCE, vinyl chloride and other VOC concentrations in indoor air, subsurface vapors, and outdoors at levels sufficient to meet regulatory requirements. The approach adheres to USEPA Method TO-14A. Additional customized features include multiplexing to allow for continuous monitoring from up to 16 locations as far as 300 m from the analyzer, evaluation of spatial and temporal concentration dynamics, measurement of differential pressure across the foundation, climatic data, delivery of data to the web, and efficient remote data management of the hundreds of data points collected each day via Cloud-based automated data-processing, visualization, alerting, and response.

Results/Lessons Learned:

Stacked time series analyses via the Cloud dashboard allowed practitioners to intuitively understand the interplay between dynamic VI controlling factors and indoor concentrations. AF estimates were derived using average indoor air concentrations during VI “on” and VI “off” conditions determined by high frequency differential pressure readings. The range in building-specific AF values was compared to published values derived from randomly-timed time-integrated results. Key findings will be presented and implications discussed.

Cliff Frescura

Affiliation

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(Group 1, Poster Board #138) Empirical Building-Specific Attenuation Factor Determination via Automated Continuous Monitoring of Chemical and Physical Parameters