Environmental Engineering Reference
In-Depth Information
should generally be designed to provide basic information regarding the contam-
inants present, their relative concentrations, possible presence (and distribution, if
possible) of NAPL, and temporal variability through periodic monitoring in selected
locations. In addition to sampling for chemical analysis, it is advisable to collect
other data for use in screening level modeling, e.g., permeability, moisture con-
tent, biodegradation assessment information. The site conceptual model should be
updated as appropriate after these additional data are collected.
If site-specific assessments of the source and distribution of soil vapors indi-
cate the potential for vapor intrusion, further characterization may be appropriate
that requires a sampling team to enter a building (i.e., “internal investigations”,
which typically include indoor air and sub-slab soil gas sampling and analysis).
While it may be tempting to conduct internal investigations as early as possible in
the process, experience has shown that in most cases, indoor air samples contain
detectable concentrations of VOCs from consumer products, building materials and
even outdoor air that are within the range of risk-based target concentrations for
several compounds (Dawson and McAlary 2008 ), in which case, it can be difficult
or impossible to interpret the relative contributions from subsurface vapor intru-
sion. Subsurface samples (soil gas or groundwater) are less likely to be significantly
affected by background contributions, and the relative proportions of the contam-
inants in the subsurface can be compared to the relative proportions of the same
contaminants in indoor air to assess consistency (indicating vapor intrusion is the
dominant contribution to indoor air VOCs) or inconsistency (indicating background
contributions are likely significant). Thus, it is usually valuable to have a reasonable
amount of data from assessing the source and subsurface pathway before conducting
internal investigations.
If the distribution of subsurface vapors has been adequately mapped, there may
be sufficient information with which to justify a “primary” investigative zone (most
likely to have vapor intrusion at unacceptable levels), a “secondary” zone (unlikely
to have unacceptable vapor intrusion, but included in the monitoring program as
a precaution to account for spatial variability and subsurface heterogeneity), and a
“tertiary” zone which would not be monitored unless nearby properties in the sec-
ondary zone are found to have unacceptable vapor intrusion. In some cases, it may
be more practical to proceed with implementing exposure controls in lieu of further
investigation. It is important to monitor the effectiveness of mitigation measures
since soil air migration might change over time. More information about vapor intru-
sion mitigation can be found in EPA ( 1993b ), Eklund et al. ( 2007 ), Folkes ( 2002 )
and ITRC ( 2007 ).
10.3 Fate and Transport Processes
Knowledge of the theory of vapor fate and transport mechanisms is essential for
interpreting the data collected during a site-specific assessment of subsurface vapor
intrusion. It is also invaluable for selecting data collection activities to focus on the
most important locations, contaminants, or parameters in order to maximize the data
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