Department of Health Seal

TGM for the Implementation of the Hawai'i State Contingency Plan
Section 7.2
SOIL VAPOR TRANSPORT MECHANISMS AND CONCEPTUAL SITE MODELS

7.2 SOIL VAPOR TRANSPORT MECHANISMS AND CONCEPTUAL SITE MODELS

7.2.1 Factors Affecting Subsurface Vapor Flow and Impacts to Indoor Air

As introduced in the previous section, understanding how vapors are generated, migrate in the subsurface and can intrude an overlying building is important for development of site investigation objectives and associated sampling plans. In theory, the rate and flux of VOC diffusion through the vadose zone is relatively simple to model (e.g., see USEPA 2004e). In practice, estimation of the upward, mass flux of vapor-phase VOCs in the subsurface and prediction of VOC concentrations in subslab soil vapor is very difficult.

Figure-7-1

Figure 7-1: Example Vapor Plume Contours and Vapor Intrusion Pathways. Vapor-phase chemicals diffuse away from a source area. Wind effects (or heating) can cause depressurization of buildings and advective intrusion of vapors. Air conditioning (AC) can over pressurize a building as fresh air is brought inside and induce an outward flow of air into the subslab space. Source: Modified from API 2005. Upward migration of vapors dominated by diffusion; advective flow limited to near vicinity (a few feet or less) of floors of under-pressured buildings.

Concentrations of VOCs in shallow or subslab soil vapor are oftentimes significantly lower than would be predicted by models based on the soil type observed in the field (see HDOH 2016, USEPA 2012). This is probably due in part to adsorption of vapor-phase VOCs to clays in the soil and permanent removal from the vapor plume, a mechanism not directly taken into account in the vapor intrusion models used to generate the HDOH soil gas (“vapor”) action levels. This is also due to the heterogeneous nature of contaminant distribution, soil type, moisture and other factors that complicate the collection of representative data. These factors highlight the need to collect soil vapor data in the immediate vicinity of potentially affected buildings as a routine part of vapor intrusion studies when general site knowledge suggest a potentially significant vapor intrusion risk.

Vapors migrate in subsurface soils primarily by diffusion from high- to low-concentration areas (Figure 7-1). Vapors diffuse much more rapidly through air-filled pore space than water-filled pore space. Advective flow of vapors caused by pressure differentials (e.g., flow from high- to low-pressure areas) can occur in the near proximity (few inches to few feet) of building floors in cases where the building is under-pressured in comparison to subsurface soils. This can be due to wind effects, changes in barometric pressure due to storms, heating of buildings (unlikely in Hawai‘i), or the use of exhaust fans in kitchens or shop areas (see Figure 7-1; see also USEPA 2004e, ITRC 2007, USEPA 2012d). Wind-induced depressurization of buildings will be the most likely cause of vapor intrusion in Hawai‘i. Wind can create a low-pressure zone on the downwind side of a building. Air pulled out of the building as a result can lead to the advective flow of subsurface vapors through cracks and gaps in the floor. This is taken into account in building and HVAC system design.

Buildings with HVAC systems (“Heating, Ventilation and Air Conditioning”) are specifically designed to minimize the infiltration of outdoor air via pathways other than the fresh air intake, in order to ensure efficiency and control costs. More likely for buildings in Hawai‘i, air conditioning will cause buildings to be over-pressured as fresh air is pulled into the HVAC system ((Roberson et al 1998; see Figure 7-1). This could induce the outward flow of indoor air into subslab soils (see also USEPA 2012d). Samples of subslab soil vapor would in turn reflect the concentration of VOCs in indoor air samples, rather than a subsurface source. This presumably explains the apparent absence of significant vapors immediately beneath slabs of air-conditioned buildings that overlie shallow, petroleum free product or heavily contaminated soil. In this case, the sudden, upward “attenuation” of deeper soil vapors in the immediate vicinity of a building slab is not attributable to biodegradation.

Note that an upward diffusion of vapors into the subslab area could also occur when the air conditioning is turned off in the night time and on weekends. This issue has not been studied in detail. In theory, this could lead to the intrusion of subsurface vapors into the building during these time periods. In practice, this is likely to be offset by the time required for deeper vapors contaminants to diffuse into the zone of advective transport. Impacts to indoor air by intruding vapors are also likely to be offset by increased impacts from indoor sources (see Section 7.7). Impacts to indoor air from both subsurface and indoor sources during periods when the building air conditioning system is not operating are generally transient in nature, with contaminants quickly removed upon restart of the HVAC system. Additional information on this topic will be found in Brewer et al. 2014, in prep.

Evaluation of risk posed to occupants should be based on air quality during normal building operating conditions (see also Section 7.10.1). More detailed sampling could be required on a site-specific basis, however, at sites considered to be of high risk for potential vapor intrusion.

Concentrations of volatile chemicals in indoor air associated with indoor sources are also likely to increase when the building HVAC system has been turned off and reach levels significantly higher than reported for typical, indoor air (see Section 7.7.2). These types of temporal changes associated with operation of the building HVAC system are important to recognize as part of a vapor intrusion investigation and to consider when determining the timing and frequency of sample collection (see Section 7.10.1). As discussed in Section 7.11, if indoor air samples are desired or required to further assess potential vapor intrusion hazards then they should be collected under normal building ventilation and operation conditions that reflect periods when the building is occupied. This more accurately reflects the potential risk to occupants of the building.

Figure-7-2

Figure 7-2: Conceptual Model of Soil Vapor Transport Including Biodegradation Process. Source: Adapted from API 2005. Note hypothetical anaerobic zone immediately beneath the building due to biodegradation of vapor-phase petroleum compounds and inadequate replenishment of oxygen.

In Hawai`i, seasonal weather variations typically include the “wet” season during the winter, and the “dry” season during the summer. The water table rises and falls accordingly. The magnitude of this rise and fall is minimal in coastal areas near sea level. In inland areas the seasonal water table fluctuation can reach ten feet or more, however. The rise and fall of the water table can create a smear zone of contaminated soil of equal magnitude, especially in the case of petroleum releases that have reached groundwater. As the water table falls and exposes this smear zone, an increase in vapor emissions can occur. As the water table rises some product may rise with it and continue to pose vapor emission hazards. A substantial portion is likely to remain trapped in the smear zone below the water table, however. This can result in a substantial reduction in vapor emissions during the wet season. The collection of deep and/or subslab soil vapor samples during both the wet and dry season is, recommended for sites where exposure of a significant smear zone could vary dramatically over the year (see Section 7.10.1).

The rise and fall of the water table with fluctuating tides could also influence the migration of vapors in the vadose zone. Indoor air could be pulled out of the building and into the subslab zone as the water table falls. The same air, or a mixture of this air and VOCs from subsurface contamination, could be pushed back into the building as the water table rises if the building was not over-pressured. This phenomenon has not been studied in detail in Hawai‘i. Small, tide-related fluctuations of the water table observed in coastal areas of Hawai‘i, typically less than one-foot, are unlikely to cause significant fluctuations in vapor concentrations due to exposure and flooding of smear zones. Tidal pumping of air into and out of a building could also help maintain a well-oxygenated zone under a building slab and help protect against significant vapor intrusion associated with subsurface, petroleum contamination.

As discussed in Section 7.10.1, consideration of tidal pumping is not necessary for general screening purposes. The collection of subslab soil vapor samples during periods of both falling and rising water table may be recommended or required, however, at sites that overlie significant, shallow contamination.

7.2.2 Preparation of Conceptual Site Models for Soil Vapor Investigations

Consideration of subsurface vapors and the potential for soil vapor intrusion should be included in an overall conceptual site model (CSM) and used to design sampling strategies. The CSM should include information on the expected subsurface geology, depth to the potential source contaminants or groundwater, and actual or potential human or environmental receptors, as well as other specific information described in Section 3. The CSM should be used to develop a general understanding of the site, evaluate potential risks to public health and the environment, and assist in identifying and setting priorities for planned activities at the site.

The CSM should reflect the representative, average subsurface conditions and building susceptibility to vapor intrusion over time and during normal building operation. This is important, because the soil gas (and indoor air) action levels are based on average exposure over a six-year time period (noncancer hazard; e.g., TPH) to thirty-year time period (cancer risk; e.g., benzene and PCE). A focus on soil vapor samples collected during periods of high water table or vapor flux assumptions during periods when a building is over-pressurized can lead to the underestimation of potential vapor intrusion hazards. A focus on subsurface data collected during periods of low water table or periods when the building is under-pressured and most susceptible to vapor intrusion could overestimate the actual risk and lead to unnecessary remedial actions. An understanding of subsurface and building conditions throughout the year as part of the CSM is therefore very important.

A simple conceptual model of soil vapor transport includes the outward diffusion of vapor-phase chemicals from impacted soil or groundwater and the potential advective flow of the vapors into an overlying building (Figure 7 1). The chemicals could migrate to and intrude residential or commercial/industrial building interiors. Common vapor intrusion pathways into buildings include basements, crawl spaces, cracks, and utility penetrations in concrete slabs. The intruding vapors subsequently mix with indoor air and the concentration of initial chemicals in the vapors is attenuated.

Figure-7-3

Figure 7-3: Complete Exposure Pathway CSM for Soil Vapor to Indoor Air.

A more detailed conceptual model of soil vapor transport might consider spatial temporal variations in subsurface conditions and building operations (e.g., daily or seasonally). Concentrations of VOCs beneath the slab of a home or building are likely to be heterogeneous (USEPA 2012d; Brewer et al. 2014, in prep). This factor and uncertainty regarding specific, vapor entry routes complicates the investigation of potential vapor intrusion hazards. As discussed in Section 7.6.2.2, the biased collection of subslab soil vapor samples from center of slabs, presumed to be the worst-case area for vapor accumulation as well as potential vapor entry points in other areas of the building (e.g., cracks in floor and utility gaps) is recommended.

The CSM could also include biodegradation processes commonly observed with petroleum hydrocarbon or volatile organic compounds (VOC) impacted soil and groundwater (Figure 7-2). The biodegradation processes include aerobic and anaerobic degradation of contaminants and potential production of additional chemicals of concern (referred to as daughter products). These conditions could change over time, as the release ages. The vapor transport of daughter products, oxygen, CO2, and in the case of petroleum hydrocarbons, methane, should be considered when assessing aerobic or anaerobic biodegradation processes.

The exposure pathway for soil vapor should be included on the CSM, which serves as the basis of an exposure assessment (see HDOH 2016). An exposure pathway is defined as “the course a chemical or physical agent takes from the source to the exposed individual”. A completed exposure pathway to a potential receptor has the following four elements: (1) a source of contamination, (2) a contaminant release mechanism, (3) an environmental transport mechanism, and (4) an exposure route at the receptor contact point with the chemicals of concern. An example of a complete exposure pathway CSM diagram for soil vapor to indoor air is provided in Figure 7-3.

For the chemicals of concern to reach a potential receptor, each of the four elements of an exposure pathway must exist and must be complete. If any of these four elements are missing, the path is considered incomplete and does not present a means of exposure under the conditions assumed in the CSM. Common pathways for vapor intrusion from the subsurface are cracks or utility penetrations through the slab or basement walls/floor, sumps with earthen floors, and drain pipes (see Section 7.7.2). Bathrooms, kitchens and utility rooms are often the primary entry points for intruding vapors.

As discussed in Sections 7.6.2 and 7.10.1, it is important that a well-thought-out CSM be prepared prior to an investigation and used to help determine the number and location of vapor collection points as well as the frequency and timing of sample collection. See Section 3 for more information on designing a CSM. See Section 13 and the HEER Office EHE guidance for details on environmental hazard evaluation. Section 7.14 discusses the use of a multiple-lines-of-evidence approach to evaluate potential vapor intrusion hazards on a site-specific basis for cases where a high risk of vapor intrusion is identified.