Environmental Fate and Transport of Solvent-Stabilizer Compounds - Environmental Investigation and Remediation - page 149

movement through sewer lines and storm drains—may provide the connection between sources of
l ammable vapors and sources of ignition.
Vapors accumulating beneath building foundations may intrude into occupied spaces, creating
an inhalation exposure hazard. The degree to which vapors may intrude into enclosed spaces
depends on the same factors as for volatilization from dry soil described in Section 3.1.2 , but also
includes factors specii c to the building-soil interface. These include the surface area of the build-
ing slab or foundation, diffusion coefi cients describing the efi ciency of vapor transport through
cracks in the slab, the thickness of the slab, air exchange rates for the structure, soil-gas l ow rates,
and other factors. The reader is directed to the authoritative references on the topic of vapor intru-
sion * ; this section addresses the intrinsic vapor transport characteristics of 1,4-dioxane and other
solvent stabilizers.
The physical and chemical parameters controlling contaminant vapor transport are source-zone
concentration, vapor pressure, vapor density, solubility, and sorption characteristics. These param-
eters determine contaminant equilibrium distribution between the vapor, sorbed, and dissolved
phases. Contaminant biodegradability and chemical stability will determine contaminant persis-
tence in each of these subsurface phases, as discussed in Section 3.3.4.
The soil characteristics governing contaminant vapor transport in a homogeneous soil include
porosity, soil moisture content, soil-air water vapor content or humidity, and soil-air l ow rates
governed by pressure and temperature gradients. In the absence of advective l ow of soil air, molec-
ular diffusion, characterized by contaminant diffusivity in air as discussed in Section 3.1.2, is the
dominant property controlling soil-vapor transport. Soil is rarely homogeneous; soil characteristics
such as stratigraphy, structures, organic matter, roots, biota, mineralogy, and density all present
challenges to the reliable estimation of soil-vapor transport.
The vapor pressure of a liquid or solid is the pressure of the gas in equilibrium with the liquid or
solid at a given temperature. Volatilization, the evaporative loss of a chemical from the liquid to the
vapor phase, depends on the vapor pressure of the chemical and on environmental conditions that
inl uence diffusion from the evaporative surface. Chemicals with relatively low vapor pressures,
high sorption onto solids, or high solubility in water are less likely to vaporize and become airborne
than are chemicals with high vapor pressures or less afi nity for solution in water or adsorption
to solids and sediments. Because most solvent-stabilizer compounds are highly soluble and have
relatively low vapor pressures, they are generally unlikely to partition into the soil-vapor phase and
migrate through the soil as vapors.
The probability that a compound occurs in the gas phase depends not only on its vapor pressure
but also on its water solubility and adsorption/desorption behavior. Therefore, even substances that
have a relatively low vapor pressure (down to 10 −3 Pa) can be found in soil vapor in measurable
quantities (Verschueren, 1996).
Vapor pressures, vapor densities, diffusivities, and relative evaporation rates for selected stabi-
lizer compounds are summarized in Table 3.1 . Appendix 3 provides a comprehensive listing of
physicochemical parameters for a longer list of solvent-stabilizer compounds. Figure 3.4 contrasts
vapor pressures with aqueous solubilities for the solvent and solvent-stabilizer compounds in
Appendix 3. In Figure 3.4, compounds plotting in the box in the upper left corner have the highest
likelihood of partitioning into the vapor phase and being transported.
3.3.4 B IODEGRADABILITY OF S OLVENT -S TABILIZER C OMPOUNDS
Information presented in this chapter focuses on laboratory studies geared toward identifying organ-
isms capable of 1,4-dioxane destruction or transformation, identii cation of degradation pathways,
* See Johnson and Ettinger (1991), Little et al. (1992), Johnson et al. (1999), American Petroleum Institute (1998), and
DTSC (2004).