Environmental Engineering Reference
In-Depth Information
sources cannot be eliminated,
therefore, vacuum and flow monitoring are
important.
Sub-slab depressurization is a containment technology, and is not designed to
remove the source of the subsurface vapors. Therefore, monitoring and maintenance
will generally be required to ensure that the systems continue to operate as designed,
as long as there is a continuing source of vapors in the subsurface.
10.6.1.2 Soil Vacuum Extraction
In some cases, it may be advantageous to control subsurface vapor intrusion by
removing the source of subsurface vapors, rather than, or in addition to blocking
the pathway to indoor air. Soil vacuum extraction (SVE) will often be the preferred
technology for this approach.
SVE is similar to sub-slab venting, but at a larger scale, with a more aggres-
sive design. SVE is generally intended to remove soil vapors from throughout
the vadose zone, accomplishing containment in the short-term and removal of the
source of vapors over time. SVE systems will generally require more infrastruc-
ture (larger blowers, water knock-out, off-gas treatment through activated carbon
filters, and sometimes telemetry and programmable logic controls), permits, regular
monitoring, and progress reporting.
10.6.1.3 Building Pressurization
Building pressurization may be as effective as sub-slab de-pressurization for main-
taining a pressure gradient across the floor slab sufficient to reduce or eliminate
subsurface vapor intrusion. For commercial and industrial buildings where HVAC
units are usually mounted on the rooftop and blowing air into the building, it may
simply be a matter of running the fans continuously, increasing the air flow rate,
and/or installing additional units. In some climates, the energy required to heat or
cool air may make this alternative prohibitively expensive. Long-term administrative
controls would be required and may be challenging to monitor or audit.
10.6.1.4 Sealing Cracks, Sumps, Sewers, and Other Potential Conduits
Just as sealants can be used to prevent water from entering a basement through
discontinuities, they may also prevent or reduce vapor intrusion. Sealants made
with current technology (urethane) are very durable and can be expected to last for
decades. Holes dilled along cracks can be used to inject sealants to facilitate good
penetration and adherence. If an industrial building is planned for re-development,
it is often a good idea to seal the entire floor, since historic releases of solvents may
have permeated the pores within the concrete. Where cracks are accessible to be
sealed, this method may result in a reduction in vapor intrusion, but the magnitude
of the reduction achieved by sealing alone will probably be less than the reduction
achieved by methods that actively manipulate pressure gradients.
Search WWH ::
Custom Search