Vapor Intrusion - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 436

a building with a basement depth similar to the depth to the water table (“wet base-
ments”), groundwater may be the only compartment beneath the building that can
practically be sampled.
In some cases, groundwater data may not be appropriate for screening the vapor
intrusion pathway. For example, if a building is overlying soils contaminated with
residuals from historic releases, groundwater quality data may not be relevant at all.
If contaminated groundwater is overlain by a fresh water lens, as shown previously
in Fig. 10.5 , and monitoring well screened intervals are too long to reflect condi-
tions at the water table, the groundwater data may provide misleading results. A
fresh water lens of a foot/ 0.3 m or more in thickness may be sufficient to mini-
mize or eliminate volatilization from the groundwater. If volatile contaminants exist
in a confined aquifer, the aquitard may act as a partial or substantial barrier to
upward migration of contaminants, in which case, vapor intrusion may be negligi-
ble. Therefore, at sites where groundwater concentrations appear to be high enough
to contribute to vapor intrusion, it may be appropriate to collect confirmatory soil
gas data to assess the degree of volatilization from the groundwater table. Transects
of shallow groundwater samples can be helpful in some cases for determining an
appropriate scope of soil gas and indoor air quality monitoring programs.
For building where the water table is very near the foundation (i.e., the “wet
basement” scenario), it may be impossible to collect sub-slab soil gas samples or
exterior soil gas samples from adjacent to the building and below the footing, and it
may therefore be necessary to rely on shallow groundwater and indoor air data for
the assessment. The relative concentrations of various contaminants in each com-
partment should be similar if the origin of the vapors in indoor air is the subsurface.
Contaminants with higher proportions in indoor air relative to other contaminants
are likely to either originate from indoor or outdoor air sources not related to the con-
taminated site, or at least have a contribution from background sources that should
be closely reviewed (Weisel et al. 2008 ).
10.5.2.2 Sub-Slab Soil Gas
Sub-slab soil gas is the gas that exists immediately beneath the floor of the occupied
structure, regardless of whether the structure is a slab-on-grade or basement design.
Sub-slab soil gas sampling has a number of advantages over sampling of other com-
partment. It is more representative of composition of subsurface vapors that may
migrate to indoor air. It is less susceptible to impacts of background sources of con-
taminants, although with barometric pressure fluctuations, it is possible for indoor
air sources to cause vapors to move from the building. Because soil gas concentra-
tions tend to be high, they are typically easier to resolve against laboratory reporting
limits than indoor air samples.
Sub-slab soil gas sampling is relatively simple and can be accomplished with an
electric hammer-drill, avoiding the need for a more-costly drilling rig. However,
sub-slab sampling has certain drawbacks. It requires an access agreement from
the building owner, and is intrusive to the extent that equipment must be brought
into the building, dust is generated and floor-coverings may be damaged, all of