Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 7.1
OCCURRENCE OF SUBSURFACE VAPOR PLUMES

7.1 OCCURRENCE OF SUBSURFACE VAPOR PLUMES

Sites where releases of volatile organic compounds (VOCs) can be of concern include commercial, military and industrial fuel facilities with petroleum storage tanks and pipelines; degreasing, cleaning or dry cleaning operations where chlorinated solvents are utilized; and agricultural operations where fumigants such as dibromochloropropane were stored, mixed or applied. The size of contaminated sites can range from a few hundred square feet associated with a small, one-time release from an underground storage tank to several acres associated with large long-time releases from fuel pipelines and aboveground storage tanks.

The emission of volatile chemicals from contaminated soil and groundwater can create a plume of vapors in the vadose zone. These plumes can adversely impact indoor air if drawn into an overlying building, a key topic of this section. Vapors emitted at the ground surface can also affect outdoor air. This issue is addressed separately under direct-exposure models for contaminated soil, however, and is considered to pose less of a threat to human health than vapor intrusion into buildings (see HDOH 2016). Vapors in vadose-zone soil could also migrate downwards and impact groundwater that has otherwise not been directly affected by the release. This has been recognized, for example, at MTBE release sites on the mainland.

The majority of subsurface vapor plumes in Hawai´i are associated with subsurface released petroleum fuels including gasoline, diesel and jet fuel. As discussed in Section 7.13, vapors emitted from petroleum fuels are evaluated in terms of Total Petroleum Hydrocarbons (TPH) and a short list of individually targeted, individual compounds including benzene, toluene, ethylbenzene, xylenes (BTEX), methyl tertiary butyl ether (MTBE, not widely used in Hawai‘i) and naphthalene (see also Section 9). Methane, a biological breakdown product of petroleum or a component of landfill gas, can also be of importance at some sites. As discussed in Section 7.6, petroleum-related vapor plumes that could pose hazards for overlying buildings are almost always associated with the presence of relatively shallow, free product in vadose-zone soil or groundwater (see USEPA 2013). Under most site scenarios, the breakdown of petroleum compounds by naturally occurring bacteria in the soil will ensure that vapor plumes rarely migrate more than 15 to 30 feet vertically through unconsolidated soil and more than one-hundred feet laterally under pavement or buildings from the source area (see Section 7.6.1).

A smaller number of subsurface vapor plumes in Hawai‘i are associated with releases of chlorinated solvents from dry cleaners (e.g., tetrachloroethene or “PCE”) or parts washing operations (e.g., trichloroethene or “TCE”). Vapors emitted from these releases are evaluated in terms of the primary product released as well as related breakdown chemicals, such as dichloroethenes or dichloroethanes and vinyl chloride. Although the volume of product released is typically much smaller in comparison to releases of petroleum fuel, the higher toxicity and in particular the greater persistence of chlorinated solvents can lead to potential vapor intrusion concerns even in the absence of free product in soil or groundwater. Dilute plumes of solvent-contaminated groundwater have, for example, been documented to travel thousands of feet downgradient of initial release areas and impact overlying homes and buildings (e.g., see API 2005, USEPA 2004e, USEPA 2012)

Both chlorinated solvents and non-chlorinated petroleum products could be present at some sites. Common examples include dry cleaning facilities that have a fuel tank associated with a boiler and/or that used Stoddard solvent during an earlier period of operation. The presence of high levels of vinyl chloride in groundwater or soil vapor at sites often indicates the presence of co-located petroleum contamination. The vinyl chloride is associated with reductive dechlorination of chlorinated solvents in the presence of petroleum. The presence of significant breakdown products in soil vapor or groundwater signifies the need to look for petroleum contamination in the same area.

Due to the inherent heterogeneity of VOCs in subsurface vapor plumes and the uncertainty of upward vapor migration from deeper areas, HDOH emphasizes the collection of soil vapor samples from immediately beneath a building slab for evaluation of potential vapor intrusion hazards (see Section 7.6.2.3). The concurrent collection and evaluation of deeper soil vapor samples is also typically recommended for heavily-contaminated properties. Data from deeper samples may indicate a need to seal cracks and gaps in floors as an added measure of protection even in cases where subslab data do not suggest a significant problem (see Section 7.14.1).