Entry
I have often been asked about the role of modeling in conducting a VIC assessment (Tier 3 in E 2600-08) and how it compares to conducting indoor air sampling. Is there anyone who would argue against the fact that the only way to really know if a problem exists is by conducting indoor air sampling?
Unfortunately, developing an acceptable indoor air sampling protocol is not as simple as it may sound. An acceptable protocol must be designed to collect representative samples. However, the problem is that experience has shown indoor air sampling results can exhibit temporal and spatial variability, and be impacted by indoor activities, building materials and operations, indoor product and chemical usage, and chemicals in the ambient air. To get a handle on all these "background" confounding factors, if it is even possible, can take considerable time and involve significant cost.
Also, it is not even possible to conduct indoor air sampling if the property involved in the real estate transaction has not yet been developed and no structures exist.
As such, there is a role for modeling, e.g., use of a Johnson-Ettinger type model or even an empirical-type model. However, understanding all of the assumptions and default values used in any model is absolutely critical. Unfortunately, there is no perfect model. While models may not be able to definitively determine absolute indoor air concentrations, they can be valuable tools to assess relative impacts, such as what might be experienced if building or soil characteristics are changed or a paved parking lot is added around a structure.
In the final analysis, it definitely makes sense to use a "multiple lines of evidence" approach. In view of all the uncertainty, it also is understandable how a client may instead choose to pre-emptively install a mitigation system.
I would be interested in hearing about your experiences with indoor air sampling and the use of vapor intrusion models for VIC assessment.
Comment
Professor, recently I'm wondering whether we can apply the principles described in the ASTM E2600 guildline to forecast the potential health risk based on the structual characteristic of the buildings, combined with the function of them(e.g. as residential building, office building or others), to be built vertically above an area heavily contaminated by BTEX. You see based on the assessment results from the prevailing RBCA_toolkit, the risk is too conservative. So now we resort to the following approaches to emend the results: Firstly, we are planing to measure the flux of the COC and then estimate the exposure concentration under different scenarios through the related models, with the corresponding risk being calculated followed. But given the lack of experience and so much uncertainty, as well as so many variables, in the measurement process, we are not so confident with the final results. So we are considering whether we can use the soil gas, the measurement of which is not so heavily affected by the surrounding enviromental factors, as the baseline value to predict the exposure concentration by mutiplying it with an attenuation factor. If this approach is defensible, how can we make reasonable decisions on the following two matters:1) the depth of soil gas sampling point; 2) the value of the attenuation factor;
Thanks in advance and look forward for your opinions
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