Theme: 09. Vapor Intrusion

Topic Area: 9d. Vapor Intrusion Risk Assessment and Site Management

Formatted Title:

The Use of Quantitative Passive Samplers to Best Determine if a Vapor Intrusion Risk is Present

Background/Objectives:

While subslab samples are collected in many regions of the United States to determine if a potential vapor intrusion risk is present, the spatial variability of subslab soil vapor concentrations compounded with the presence of utilities penetrating buildings create challenges to this approach being used as a best practice to conclude definitvely whether vapor intrusion may occur. To know the concentrations of VOCs in indoor air, it is best to sample the indoor air. However, research completed over the past decade at universities, as well as data collected on commercial projects, has clearly demonstrated that the temporal variability of indoor air concentrations can vary by orders of magnitude in both long and short periods of time. Therefore, there is a growing trend by regulators to require data be collected over an extended time period (e.g., 10 to 14 days). The collection of daily 24-hour samples over these extended time periods is cost prohibitive; therefore, the use of passive samplers to collect long-duration, time-integrated samples is being advanced and adopted as a best practice to reliably determine average concentrations of VOCs in air and confidently determine whether mitigation is required.

Approach/Activities:

The use of adsorbent samplers to passively sample air over days or weeks provides long-duration average concentrations to assess health risks, which overcomes the challenges from temporal variability and episodic occurrences of vapor intrusion. Passive samplers are low-profile and are more readily adopted by building occupants and easier to manage by field technicians than evacuated canisters (Method TO-15). An ongoing community sampling program involves the collection of indoor air samples at hundreds of homes annually by use of passive samplers, with uptake rates that were verified to be linear out to 26 days by a robust study completed by an independent third party. The passive samplers provide a 26-day, time-weighted average concentration that represents with one sample more than 25% of the entire winter season. The prior sampling program that used inert stainless steel evacuated canisters to collect discrete, 24-hour point-in-time samples, was found to not represent the actual exposure risks of the building occupants; therefore, homes that even recorded non-detects were not removed from the monitoring program. By collecting long-duration samples with parts per trillion by volume (pptv) reporting limits following US EPA Method TO-17, greater confidence was achieved that the health risks were properly assessed and ongoing monitoring and mitigation strategies could be determined.

Results/Lessons Learned:

Sampling indoor air identifies the intrusion of VOCs whether through the building slab by diffusive and/or advective forces or through utility pipes that create a direct VI pathway into the building. The use of passive samples to collect samples over 26-day periods has produced data that are of higher confidence for determining health risks than when 24-hour samples were collected and regulators are now more confident in determining which homes require no further sampling and which require mitigation. Passive samplers additionally have gained acceptance because they are lightweight, easy to transport, and are a more sustainable approach than the use of bulky, evacuated stainless steel canisters. The simplicity of use of these devices results in less errors in the field and an easier to clean sampler that can eliminate cross contamination issues that are known to be common for evacuated canisters. In addition, in the most recent round of sampling at the ongoing community sampling program, the simplicity-of-use of these devices allowed residents the option of hanging the samplers in their own homes to minimize the contact with others when virus transmission concerns were present. The ability to have residents receive samplers at their doors and hang them in their homes following documented procedures and conditions will increase the number of homes that are able to be sampled in both ongoing and future sampling programs by overcoming the stigma of having someone enter ones home.

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Steven Thornley