A question I have frequently been asked at vapor intrusion training courses is why in a real estate transaction after identifying a pVIC in Tier 1-Tier 2 screening under E 2600-08 does it often make sense to by-pass Tier 3, VIC Assessment, and proceed directly to Tier 4 pre-emptive mitigation.

The answer to this question gets to the heart of the problems with the tools that can be used in Tier 3. Fundamentally, the problem is that no matter what lines of evidence you decide to pursue, there is significant variability and uncertainty. There is no question about this being the case with modeling based upon soil gas or groundwater concentration data. Even indoor air testing is not simple and straightforward. Indoor air measurements must distinguish between chemical contributions from background sources and from contaminated soil and/or groundwater. Unfortunately, there often is a high degree of variability in both sets of measurements. Actual indoor air test results also have a high degree of variability, including both temporally and spatially. Indoor air results can be impacted by weather, which is highly variable, structure operating characteristics and mechanical system operation, both of which are highly variable. Clearly, trying to confirm whether a VIC truly exists in Tier 3 is no easy task and can take considerable time and involve significant expense. Such time generally is not available in a real estate transaction, and the expense can be greater than the cost of mitigation. This is why in my view it often makes sense to by-pass Tier 3 and proceed directly to Tier 4.

I would be interested in hearing about any real estate transaction cases where it made sense to proceed to Tier 3, rather than by-passing it. Anyone?