Vapor Intrusion - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 444

g/m 3 (roughly 200 ppbv for many VOCs), which is considerably higher
than fixed laboratory reporting limits (e.g., 0.5 ppbv for US EPA Method TO-15).
Recent advances in mobile laboratory technology have enabled TO-15 analyses
to be performed by mobile laboratories, provided strict QA/QC protocols are fol-
lowed. Mobile laboratories offer the advantage of real-time information, which can
be used to guide the scope of site-assessment activities and in many cases pro-
vide adequate site characterization in a single mobilization, instead of multiple
phases.

of 1,000 μ

10.6 Mitigation

Subsurface vapor mitigation systems may be implemented either because concen-
trations are high enough to pose a potential risk or as a precaution where Risk
Management decisions are intentionally conservative. This section discusses the
various options available and the expected effectiveness for each. In practice, mit-
igation measures that must be added as a retro-fit to an existing building will
generally be different than measures which can be integrated into the design and
construction of a new building. So this section is presented in two subsections,
accordingly.

10.6.1 Methods/Technologies for Existing Buildings

10.6.1.1 Sub-Slab De-Pressurization

Sub-slab depressurization is the most common form of control for subsurface vapor
intrusion. In simple terms, a vacuum is applied to the region beneath the floor slab,
maintaining a pressure differential sufficient to prevent vapor intrusion. A concrete
floor slab is usually underlain by granular fill for structural support, and the granular
fill is usually sufficiently permeable to facilitate extraction of sub-slab soil gas and
propagation of a vacuum. As a result, a single extraction point connected to a fan
of approximately 50-150 Watts is usually sufficient to achieve reductions in indoor
air concentrations by 2 or 3 orders of magnitude. Monitoring to confirm the effec-
tiveness usually involves demonstrating that the vacuum is propagating across the
floor to some degree (typically 6-9 Pascals, ASTM 2001 ). These systems are often
identical to radon mitigation systems. The fans are rated for continuous duty and
last several years between replacements (EPA 1993b). Figure 10.9 shows a typical
design. Figure 10.10 shows performance monitoring data demonstrating a reduction
of 3 orders of magnitude in concentrations of 11DCE after initiation of sub-slab
depressurization (Folkes 2000 , 2002 ). Concentrations of several other contaminants
were not affected, which is a clear indication that the source of these other vapors
was background contributions. These figures also point out that verification mon-
itoring by indoor air sampling and analysis is problematic, because background