Vapor Intrusion - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 419

value. Theoretical considerations should be the backbone of the conceptual model
and any mathematical models. Data will help to identify “what” is happening, but
theory is required to understand “why”.
Mass transfer of VOCs from water to air has been extensively studied for surface
water, but less so for groundwater. In either type of system, mass transfer is viewed
as occurring over four layers: bulk liquid, liquid at a boundary layer between the two
phases, air at the boundary layer, and bulk air. Mass transfer occurs by transport from
the bulk liquid to liquid at a boundary layer, phase transfer to air at the boundary
layer, and then transfer of the gaseous constituent from the air boundary layer to
the bulk atmosphere. The overall mass transfer rate is limited by the slowest step
in the process. In above ground systems, this typically is the transfer from the bulk
liquid to the liquid at the boundary layer. In subsurface systems, however, there are
additional considerations, such as vapor transport through water-unsaturated porous
media (including the capillary fringe above a groundwater table), phase partitioning
during transport, and biodegradation as described in more detail below.
10.3.1 Phase Partitioning
Phase partitioning calculations can be performed to estimate the concentration in
any one of the phases (soil gas, pore water or solid phase) from the concentration
in another phase (i.e., a soil gas sample), or the sum of all phases (i.e., a bulk soil
sample), providing the total pore water content and fraction of organic carbon (Foc)
of the soil are known. These soil properties and bulk soil VOC concentrations can
be highly variable on small scales (i.e., the scale of typical soil sample volumes), so
partitioning calculations generally reflect this variability (Davis et al. 2005 ; Tillman
and Weaver 2007a ). Therefore, it is generally best not to try to calculate soil gas
concentrations from bulk soil concentrations, but rather to measure soil gas concen-
trations directly (Provoost et al. 2009 ). Many regulatory guidance documents do not
consider screening the vapor intrusion pathway using soil VOC concentrations for
this reason.
The maximum soil vapor concentration that can be achieved in immediate prox-
imity to a NAPL can be calculated as the ratio of the vapor pressure to the total
pressure, which is essentially atmospheric pressure. This can be thought of as a sat-
uration limit in air. If the NAPL consists of a mixture of contaminants, Raoult's
Law can be used to calculate the maximum soil vapor concentration, which requires
measurement or estimation of the mole fractions of the contaminants in the mixture.
Depending on the complexity of the mixture, variations from this ideal behavior
may be significant.
Measured groundwater and soil gas concentrations do not always show the ratio
predicted by Henry's Law (Provoost et al. 2008 ). This is partly because groundwa-
ter samples tend to be a mixture of water from different depths along the screened
interval instead of being only from just below the water table, and it is also partly
because of rate-limiting mass transfer across the capillary fringe and the fact that
the subsurface is not a closed system. Therefore, it is usually advisable to collect