Theme: 09. Vapor Intrusion

Topic Area: 9a. Advances in Vapor Intrusion Investigations

Formatted Title:

How Real-Time Indoor Air Measurements Can Help Show Dynamic Vapor Intrusion

Background/Objectives:

The South Australian Environment Protection Authority (SA EPA) administers a program that investigates and manages “orphaned” site contamination where the original polluter is not known or unable to be regulated under the relevant legislation. Since 2015, the SA EPA has been undertaking assessment of legacy chlorinated hydrocarbon contamination in groundwater associated with historical industrial and waste management practices in the Adelaide suburb of Beverley, South Australia. Given the volatile nature of the contamination, which includes trichloroethene (TCE), there is not only an impact on users of groundwater but it also poses a vapor intrusion risk to the occupants of a number of residential properties. The subject dwelling was a single-storey residence with crawlspace and a slab-on-ground extension and located in an area with high TCE concentrations in shallow groundwater. Vapour intrusion modelling using the Johnson & Ettinger Model predicted indoor air TCE concentrations up to 130 µg/m³ and exceeding the safe level of 2 µg/m³.

Approach/Activities:

The SA EPA engaged Contaminated Site Investigations Australia (CSI Australia) to undertake a vapour intrusion assessment using a portable gas chromatograph (GC) that applies U.S. EPA Method TO-14 with electron capture and photoionisation detector. The GC first mapped the air quality within a number of rooms and crawlspaces throughout the property using discrete ambient air measurements. The GC was then set up for automated continuous sampling of seven rooms and one subfloor crawlspace over a period of 24 hours.

Results/Lessons Learned:

The dwelling had a footprint of approximately 200 m², of which 70% comprised timber floorboards overlying a crawlspace with a dirt base. The remaining 30% of the building footprint comprised tiled flooring on concrete slab foundation. Discrete sampling in 10 rooms and passages indicated indoor air TCE concentrations up to 51 µg/m³, with automated continuous sampling over 24 hours measuring mean TCE concentrations up to 28 µg/m³. Both the discrete and continuous sampling indicated higher TCE concentrations in samples from rooms with a crawlspace floor construction when compared to rooms with slab floor construction, suggesting the majority of vapor intrusion was likely occurring through flooring in crawlspace rooms.

The continuous sampling indoor air TCE results, coupled with concurrent ambient air temperature measurements, also revealed a significant dynamic response to a low-pressure weather system that passed over Beverley during the sampling period. Indoor air TCE concentrations were observed to spike during the early hours of the morning with a peak (maximum of 109 µg/m³) more than four times higher than the mean TCE concentration in the same room during the monitoring period. Furthermore, the spike in indoor air TCE concentrations coincided with a significant reduction in subfloor (crawlspace) air TCE concentrations. This was attributed to significant changes in the temperature differential (difference between inside and outside ambient temperatures) as the outdoor air temperature had dropped from 37 to 18 degrees Celsius overnight whilst the indoor air temperature remained relatively stable (24 to 26 degrees Celsius).

These observations were considered evidence of the “stack effect” phenomenon where warm, low-density indoor air drives upwards escaping through ceiling gaps and being replaced by air from lower in the building; in this case from the high TCE concentration crawlspace air. While this phenomenon is known to occur in cold climates where significant indoor heating occurs, in this case the temperature gradient was instead driven by changing weather conditions and building operation.

The use of continuous sampling allowed for real-time measurement of volatile contaminants at the exposure point and higher resolution of conditions under a dynamic vapor intrusion scenario when compared with conventional time-weighted sampling methods. The assessment approach also showed significantly more vapor flux into rooms with crawlspace floor construction allowing for design and installation of a customized vapor mitigation system that successfully reduced indoor TCE concentrations to safe levels.

Dane Egelton

Affiliation

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(Group 1, Poster Board #137) How Real-Time Indoor Air Measurements Can Help Show Dynamic Vapor Intrusion