Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 7.4
SOIL VAPOR INVESTIGATIONS

7.4 SOIL VAPOR INVESTIGATIONS

The following subsections discuss the collection and analysis of soil vapor and indoor air samples. Although the guidance presented is anticipated to apply under most site scenarios, issues such as sample location and depth, sample collection timing and frequency, collection of indoor air samples, etc., will necessarily be site-specific and should be discussed with the overseeing HDOH project manager.

Soil vapor samples, including samples collected immediately beneath building slabs, are collected following the discovery or suspected presence of volatile chemicals in subsurface soil or groundwater. Data are used for general site-characterization purposes and/or to assess vapor intrusion risk. Typical site investigation objectives include: 1) Characterization of in situ vapor plume conditions, 2) Assessment of potential vapor intrusion risks, 3) Assessment of worker-related environmental hazards in locations where soil vapor may accumulate (e.g., utility conduits or vaults beneath foundations, roadways and caps, 4) Development of remedial actions and 5) Monitoring or confirmation of remedial actions. Indoor air samples are collected as needed to further assess vapor intrusion risk and link or negate identified impacts to a subsurface source. Soil vapor data can also be used to assess potential impacts to groundwater posed by downward migrating vapors or volatile chemicals dissolved in downward migrating leachate (refer to HDOH 2017b).

The types of soil vapor samples collected and subsequent use of the data can vary based on the objective(s) of the site investigation. As discussed below, samples collected from multiple, “discrete” points beneath a building or in open areas and representing very small volumes of vapor (e.g., one to six liters) can be useful for identification of large-scale, vapor plume patterns. Reliance on individual sample points to identify plume boundaries or assess vapor intrusion risk is complicated, however, by the inherent variability of VOC concentrations in vapors at this small scale. The collection of “Large Volume Purge (LVP)” samples, representing thousands of liters of vapor, is recommended when feasible in order to improve data reliability (Section 7.8.5).

Testing of soil vapor is carried out through the collection of “active” or “passive” vapor samples from multiple points within the targeted investigation area and comparison of the resulting data to HDOH Environmental Action Levels (EALs) for vapor intrusion risk (HDOH 2017a). “Active” samples are collected by drawing vapor into canisters under a vacuum (Section 7.8.1) or by drawing vapor through a sorbent tube (Section 7.8.2). “Passive” samples are collected by burying and then retrieving and testing sorbent media at multiple points within the investigation area (Section 7.8.3). Small-volume, active samples(e.g., 1-6 liters) that minimize disturbance of a vapor plume and/or passive samples are used to characterize undisturbed, in situ subslab vapor conditions. Large-volume, active sample data are used to more reliably assess actual vapor intrusion risk. Although useful for general screening purposes, note that data for small-volume samples, both active and passive, are in theory not directly comparable to HDOH (2017a) action levels for vapor intrusion risk. The action levels more strictly apply to the mean concentration of a VOC in very large volumes of vapor assumed to intrude a building over many years, amounting to millions of liters of vapor per year (refer to Section 13.2; see also Brewer et al. 2014).

The use of LVP vapor sampling methods is recommended for more direct evaluation of vapor intrusion risk. This approach allows for a very large, risk-based volume of vapor to be represented by a single, active soil vapor sample (Section 7.8.5). The resulting data will thus be more directly representative of the large volume of vapor predicted to intrude into a building on a given day, for example the 3,000-liter, default, assumed daily vapor entry rate for buildings in Hawai´i discussed in Section 7.5.5. This method is currently most widely applied to the collection of subslab vapor data. The collection of deep LVP samples from areas with a thick vadose zone is feasible for soils with a relatively high vapor permeability, provided that monitoring for leakage to outdoor air is carried out. The collection of shallow (e.g., <25 ft) LVP samples from open (uncapped) areas will be hindered by potential downward leakage of outdoor air into the sampling train. In these cases, continued reliance on small-volume soil vapor sample data is still necessary. Collection and comparison of replicate sets of small-volume sample data can assist in understanding the representativeness of a single data set. Data from deeper strata are, however, less likely to be representative of a vapor plume that might form under the base of an overlying building after the effects of degradation and diffusion into a subslab advective zone are taken into account. This is especially true for nonchlorinated, hydrocarbon compounds associated with releases of gasoline and diesel fuels, where localized variability in degradation could result in a highly heterogenous vapor plume.

The information provided in this section is intended to apply to sites in the State of Hawai`i where soil vapor and, if required, indoor air samples are collected, whether the evaluation is being conducted voluntarily by private individuals or corporations or under one of the state’s environmental remediation programs. This guidance is intended to provide a technically defensible and consistent approach for the collection and evaluation of soil vapor or indoor air samples. However, this guidance is not regulation and is only meant to provide a clear technical framework for collecting and evaluating soil vapor or indoor air samples. The information contained in this guidance is not intended to exclude technically equivalent alternate approaches or methodologies that may exist.

This guidance does not address safety or hazard mitigation efforts to prevent fires or explosions resulting from the accumulation of hazardous vapors (i.e., methane); however, methane concentrations should be monitored to determine whether these hazards exist. A brief discussion of methane hazards and additional reference documents is provided in Sections 9 and Section 13 of this guidance. Emergency or immediate response actions by qualified responders should be completed prior to the initiation of a soil vapor or indoor air sampling event. If the results from the soil vapor or indoor air sampling event indicate that there is an immediate concern for human exposures to vapor phase chemicals, then emergency response or interim actions are typically implemented as required under state and federal regulations.

The HEER Office recommends that Soil Vapor or Indoor Air sampling work plans be submitted for review and approval prior to the collection of soil vapor or indoor air samples in Hawai`i. The work plan should describe the purpose and rationale for the soil vapor or indoor air sampling, targeted chemicals of concern, sample locations and depths, sample collection protocols, and analytical methods. A discussion of targeted chemicals of concern for petroleum releases is provided in Section 7.13.1.2 (see also Section 9). Work plans should be developed following the systematic planning approach and guidelines outlined in Section 3. Information on the recommended format and general content of investigation work plans is included in Section 18.