The following fate and transport mechanisms explain the behavior of PHC vapors and describe how this behavior affects the PVI pathway:
An understanding these properties can help to explain the distribution of vapors at a site, evaluate multiple lines of evidence to determine completion of the PVI pathway, and refine the CSM. These processes are described in this appendix, with further discussion in Appendix C, Chemistry of Petroleum.
Phase Partitioning
The separation of fuel constituents into soil, water, and air-filled soil porosity. Phase partitioning is most easily and accurately determined by direct sampling.
PHCs are present in the subsurface partition between the solid, liquid, and gas phases. Partitioning equations are used to calculate the relationship of chemical concentrations in these different phases. The common partitioning equations used are Henry’s law (which relates water and vapor concentrations), Raoult’s law (which relates LNAPL and vapor concentrations), and linear sorption isotherms (which relate sorbed and aqueous concentrations). Partitioning calculations can be used to estimate the gas-phase concentration from a bulk soil or groundwater concentration. Note, however, that uncertainties and broad assumptions are associated with these calculations and related models that estimate soil gas concentrations from soil or groundwater data (McHugh and McAlary 2009). If partitioning information is needed, direct sampling of soil gas is generally recommended to avoid concerns associated with the partitioning equations.
The composition of PHC vapors in the subsurface depends on both the original chemistry of the fuel and the effects of weathering. Weathering occurs primarily from three processes: volatilization, dissolution into water, and biodegradation, all of which significantly change the composition and reduce the mass of vapors potentially reaching receptors (Potter and Simmons 1998). Lower molecular weight constituents and those with higher vapor pressures preferentially volatilize from the source, thereby reducing the concentrations of these components in the source. Additionally, more soluble constituents, such as aromatic hydrocarbons (which include BTEX), preferentially partition into water and reduce the source concentrations of these constituents. Biodegradation is further discussed in
Diffusion
The dominant transport mechanism of vapors in the vadose zone, defined by movement of vapors from areas of higher concentrations to lower concentration.
Diffusion is the movement of a chemical from an area of higher concentration to lower concentration. Diffusion is typically the dominant transport mechanism for vapors in the vadose zone.
The diffusive mass flux of a vapor in the soil is described by Fick’s law. Diffusion occurs in both the aqueous and gas phases. The diffusive mass flux is directly proportional to the soil vapor concentration gradient; therefore, higher soil vapor concentrations result in higher flux. Additionally, an effective diffusion coefficient is used to integrate tortuosity of porous media and the diffusivity of the gas phase and soil moisture.
In addition to the concentration gradient, the degree of water saturation also affects the rate of the diffusive mass flux through the vadose zone. Diffusion coefficients in water are typically about four orders of magnitude lower than the diffusion coefficients in air, so as the soil becomes increasingly saturated, vapor flux decreases. Therefore, the capillary fringe (a thin layer of highly saturated soil in the vadose zone) can often reduce the overall vapor diffusive flux from groundwater to ground surface. Conversely, dry soils provide numerous air-filled pore spaces that allow for appreciable rates of diffusive transport.
The site-specific stratigraphy can also significantly affect the upward diffusion of contaminant vapors from the source and the downward diffusion of O2 from the atmosphere due to changes in soil permeability and soil moisture content. Under dry conditions, diffusive transport can be similar for soils with a broad range of textures and grain sizes. Additionally, work presented by Carr (Carr, Levy, and Horneman 2010) illustrates how layers of saturated fine-grained soils are associated with significant reductions in the concentration of chlorinated vapors.
Biodegradation Rates
The rate of biodegradation typically exceeds diffusive transport rates for PHC vapors.
A notable feature of aerobic biodegradation of PHCs in soils is the short acclimation time for this process, which can be measured in hours and days (Turner et al. 2014). The acclimation time is the time required for the microbial community to start consuming PHCs after the initial introduction of these chemicals. This short acclimation time indicates that PHC biodegradation is a common physiological trait of soil microorganisms.
Biodegradation is the breakdown of organic chemicals, including PHCs, by microorganisms. This process generally happens concurrently with and limits the diffusion of vapors through the vadose zone. Microorganisms that biodegrade PHCs are ubiquitous in most subsurface soils. Although PHCs can be biodegraded in the absence of O2, the most rapid rates of biodegradation typically occur under aerobic conditions. The vadose zone above an area contaminated by a petroleum release is normally an aerobic environment in which oxygen can be readily replenished from the atmosphere. PHC vapors are typically, but not always, fully biodegraded in the vadose zone because rates of PHC vapor biodegradation usually exceed the rates of petroleum transport via diffusion.
Despite the general reliability of aerobic biodegradation in reducing PVI, several environmental factors can slow this process. The most significant factor is the availability of O2, which is a necessary electron acceptor and enzyme reactant in the aerobic biodegradation of PHCs. Low permeability soils (for instance, those with low porosity or high moisture content) may limit the recharge of O2. Large building foundations (25 feet or more from the center to the edge of the slab) may also restrict recharge of O2 (MIDEQ 2012; CRC CARE 2013; Knight and Davis 2013). Furthermore, O2 concentrations may also be limited by its consumption because of biodegradation itself in areas of high PHC concentrations, such as near LNAPL sources. In the vadose zone, O2 present at greater than 1% supports aerobic biodegradation (Abreu and Johnson 2006). Note that a number of state regulations and guidance documents use O2 levels greater than 2–4% by volume to confirm that conditions are suitable to support aerobic biodegradation.
Acclimation of Microorganisms
Microbial communities start consuming PHCs within hours or days.
In the absence of O2, anaerobic microorganisms can use other electron acceptors to support PHC biodegradation. Anaerobic biodegradation of PHCs is typically slower than aerobic biodegradation, and the rates of biodegradation in the presence and absence of O2 can differ substantially.
Another factor limiting biodegradation is that microbial processes depend on moisture, so appreciable biodegradation rates occur only in the presence of moisture. In most vadose zone soil profiles, sufficient moisture, nutrients, and O2 are present and do not limit microbial PHC biodegradation. However, in more extreme circumstances (such as arid environments), insufficient moisture or limited nutrients can potentially limit PHC biodegradation, even if sufficient O2 is available.
Cold climates do not necessarily decrease the potential for biodegradation. Although biological processes generally slow in colder temperatures, specific PHC-consuming microorganisms thrive in temperatures ranging from 20ºC to 0ºC (Margesin and Schinner 2001). Additionally, O2 appears to be readily available in the subsurface in cold climates and biodegradation has been observed in arctic soils at subzero temperatures (Hers et al. 2011; Rike et al. 2003).
Methane Production
Methane is produced when O2 is depleted in the presence of high PHC concentrations or large source areas, or by breakdown of petroleum products containing ethanol.
When PHCs are present at sufficiently high concentrations or in large source areas, O2 and other electron acceptors may become depleted. PHCs may then be biodegraded through the activities of methanogenic microbial communities. Because methane is not a component of gasoline or other liquid hydrocarbon products, the presence of methane indicates that insufficient O2 is available for aerobic PHC biodegradation.
The presence of methane can also further affect PHC biodegradation, because methane itself can be readily biodegraded under aerobic conditions. The consumption of O2 for methane biodegradation can limit the amount of O2 available for biodegradation of other hydrocarbons. O2 levels can also be reduced in the presence of ethanol by a similar mechanism, resulting in further methane production. However, the effect of methanogenic degradation of ethanol in this context is limited in fuels containing 10% or less ethanol by volume (Wilson, Weaver, and White 2012b; Wilson et al. 2013).
The chemical structures of PHCs and their physicochemical properties can also influence their biodegradation. Generally, for PHCs dissolved in the water phase, microorganisms biodegrade n-alkanes (straight-chain alkanes) more rapidly than cyclic and aromatic compounds, and biodegrade shorter chain n-alkanes more slowly than longer chain n-alkanes (Alexander 1977). The structure of the chemical (for example, more branching) and the presence of specific substituents or functional groups (for example the ether group on methyl tert-butyl ether, or MTBE) can also strongly affect biodegradability. Another important factor that affects the biodegradability of a chemical based on its structure is the air-to-water partitioning coefficient. For petroleum chemicals, because the air-to-water partitioning of aromatic compounds is less than n-alkanes, a greater fraction of the aromatic compounds are partitioned into water, are more readily available to be biodegraded, and, therefore, may be more significantly attenuated by microbial biodegradation than n-alkanes (DeVaull 2007).
A summary of the effects of chemical structure on aerobic biodegradation rates is presented in
Pressure gradients between the building and subsurface can result in advective transport of subslab vapors to indoor air through a variety of potential entry points in the building foundation. Entry points include preferential pathways such as utility penetrations and lines, expansion joints, floor drains, and sumps, as well as structural defects such as cracks in the floor and foundation.
Pressure gradients between the air inside a building and the subsurface can be caused by several processes:
Typically, pressure differentials between the building and the subsurface are relatively small (a few to a few dozen pascals), so the zone of influence of the pressure fields associated with building-induced advective flow does not penetrate far beneath the base of the building. Pressure gradients can differ as a result of building construction; for example, the stack effect may be more significant for taller buildings. Pressure gradients may also fluctuate for a variety of other reasons, such as HVAC operations, barometric pressure, and open windows.
Mechanical ventilation systems and weather-driven increases in barometric pressure can cause overpressurization of the building and induce VOC migration from the building downward into the subslab soil. Thus, mixing of subslab soil gas and building air takes place not only inside a building, but also in the fill materials and native soils beneath the lowest floor. Consequently, the presence of a volatile contaminant beneath and inside a building may not be sufficient as a sole line of evidence that subsurface VI is occurring. Absent additional evidence, contamination of internal air and external soil gas from a contaminant source inside the building may be equally plausible.
Intruded vapors can mix and dilute with indoor air. HVAC systems may rapidly distribute intruded vapors throughout the building. Dilution of subslab soil vapor concentrations is characterized by the building ventilation rate, expressed as air exchanges per hour.