Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 7.5


The previous section outlined factors that could control the concentration of vapors in the subsurface and immediately beneath the slab of an overlying building. As discussed, the collection of representative soil vapor samples for evaluation of potential vapor intrusion hazards is critical. Geologic factors that control vapor concentrations in the subsurface and temporal variations that control vapor concentrations over time must be considered when setting investigation objectives and collecting samples. Recommendations on the location, timing and frequency of soil vapor sample collection are presented in the following sections.

Data associated with discrete soil vapor samples are typically extrapolated over large distances and depths between individual sample points. This approach has been shown to be fraught with error in soil investigations due to an inability of a small number of individual samples to adequately capture the heterogeneity of contaminants in soil (see TGM Sections 3 through Section 5). This typically results in an underestimation of representative concentration of a chemical in soil within a targeted, “Decision Unit” area. The effect of subsurface heterogeneity on the representativeness of soil vapor samples has not been well studied but can be reasonably assumed to be less than recognized for contaminants in soil, due to an assumed more efficient distribution of vapor-phase contaminants by diffusion through soil pore space. This would in theory allow the mean concentration of VOCs in soil vapor for a targeted area to be determined by a fewer number of samples (or “increments”) than for soil.

With respect to vapor intrusion, the ultimate objective of an investigation is to estimate the average concentrations of VOCs in soil vapor that will or could enter a building over a sustained, exposure duration of six years (noncarcinogens; e.g., TPH) to thirty years (carcinogens; e.g., benzene and PCE). This requires the collection of soil vapor samples from the area of the subsurface, vapor plume most likely to contribute to vapor intrusion over the assumed, six- to thirty-year exposure duration. As is the case for soil, the objective of the investigation is generally not to identify the maximum concentration of VOCs present in soil vapors (i.e., acute exposure), although this may need to be considered on a site-specific basis. The use of short vapor point screen lengths in an attempt to identify worst-case, non-representative “hot spots” (or “cold spots”) is not strictly appropriate, in the same manner that small discrete soil samples are not representative of long-term exposure to contaminants in soil (see Section 3). The average over the assumed exposure area and duration is the goal. This would ultimately apply to the average concentration of a chemical in the indoor air.

The relatively small number of samples collected in the field, and the lack of well-thought-out Decision Units in soil vapor investigations typically means that the maximum concentration of a chemical detected in samples is used for comparison to soil gas action levels and decisions regarding the need for additional investigation or remediation (see Section and Section 7.14). This approach, although currently accepted by HDOH, is reminiscent of traditional soil investigation practices that have since been demonstrated to miss large areas of contamination and underestimate potential direct-exposure and other risks. Additional guidance on the collection of representative soil vapor samples with respect to long-term, vapor intrusion concerns is currently being developed by both the HDOH and other environmental agencies and private entities in the US. Updates on the optimal number and volume of soil vapor samples will be incorporated into the TGM as available