Vapor intrusion:
A threat to your site?
A threat to your site?
By Thomas Dong, REA
According to the U.S. Environmental
Protection Agency (EPA), more than 374,000 sites nationwide may
need to demonstrate to regulatory agencies that vapor intrusion
does not pose a significant risk to human health. Common sites
include drycleaners, gas stations and other properties
owned/occupied by industries, municipalities, utilities,
federal agencies, developers, and private owners.
The good news is that, with proper
planning and appropriate technical guidance, vapor intrusion at
these sites can be successfully mitigated.
Consequently, many property owners have
had to make two key determinations: a) whether or not vapor
intrusion is an issue at their sites; and b) which steps should
be taken to mitigate problems to ensure site safety for the
environment as well as current and future users.
How dangerous is vapor intrusion?
Vapor intrusion occurs when volatile
organic compounds (VOCs) in soil and/or contaminated
groundwater volatilize, or emit vapors, and migrate through
subsurface soil and into air spaces of overlying buildings.
These vapors constitute a potential threat of exposure through
inhalation for occupants of affected buildings.
Examples of VOCs include petroleum
products such as gasoline, and solvents used for dry-cleaning
and industrial processes. Vapor intrusion can involve petroleum
spilled or leaked from underground storage tanks at gas
stations. Unfortunately, in some cases, chemical and petroleum
releases are not discovered until after contamination has
migrated through the soil.
Health effects from chemical exposure vary
based on individual constitution and chemicals involved. When
airborne chemicals accumulate indoors, some people experience
eye and respiratory irritation, headache, and/or nausea.
In extreme cases, VOCs can amass in
dwellings or occupied buildings to levels that may pose
near-term safety hazards or acute health effects. Health
officials are concerned about low-level chemical exposure over
many years, as this may raise a person’s lifetime risk
for developing cancer.
Evaluating vapor intrusion
The potential for vapor intrusion can
generally be assessed by collecting soil gas or groundwater
samples inside or beneath buildings. Because indoor air quality
changes from day to day, one round of sampling may not reveal a
problem. Collecting a number of indoor air samples over a
specified period may prove helpful.
A systematic approach is recommended for
evaluating the potential risk of vapor intrusion at a
particular site and, if required, programs to mitigate exposure
should be developed. Several common strategies are outlined
below:
1. Site screening and modeling. One or
multiple sites are screened for the potential to cause vapor
intrusion. This either eliminates unnecessary testing or
prioritizes action at sites where a potential for contamination
may exist.
2. Pathway and plume evaluation. When site
screening indicates a potential for vapor intrusion, subsequent
field investigations are necessary. Identifying and analyzing a
site’s vapor intrusion pathway — the route of
migration from subsurface to interior space — can help
owners and operators evaluate whether vapor intrusion is an
issue at their site.
3. Soil vapor samples. Since a variety of
VOC sources may be present at some sites, testing will not
necessarily confirm that VOCs in indoor air derive from
contamination in nearby soils. Instead, soil vapor samples may
be taken from beneath a building’s foundation (called
sub-slab samples) to see if vapors have reached the building.
4. Source identification. Great care must
be exercised when conducting indoor air tests to ensure that
miscellaneous background sources are properly considered.
Mitigation
The most common mitigation system is a
flexible membrane liner installed beneath the foundation of a
building, and combined in some cases with a passive venting
system to allow vapor build-up to be evacuated. If properly
installed, a membrane liner can mitigate the effects of vapor
intrusion and reduce indoor air action levels.
Why be proactive?
Potential costs and liabilities associated
with vapor intrusion impacts can be high. Being proactive
allows site owners and operators to remain in control of the
process from the beginning, thereby saving time and money.
The benefits of a proactive approach
include:
1. Development of defensible technical
arguments.
2. Reduction of liability and risk.
3. Smoother real estate transfers. A
seasoned environmental consultant specializing in vapor
intrusion issues can help walk owners through the steps
necessary to achieve these benefits.
EPA providing guidance
Vapor intrusion is a rapidly developing
field of science and policy. The EPA’s Office of Solid
Waste and Emergency Response (OSWER) has issued a draft
guidance document that provides current technical and policy
recommendations on determining if a vapor intrusion pathway
poses an unacceptable risk to human health at cleanup sites.
This guidance document:
• Replaces the draft Resource
Conservation and Recovery Act (RCRA) Environmental Indicator
(EI) Supplemental Guidance for Evaluating the Vapor Intrusion
to Indoor Air Pathways (December 2001).
• Does not supersede state guidance.
• Is suggested where structures are
within 100 feet of contaminated soil/groundwater.
• Consists of a step-by-step
evaluation process.
• Contains multiple “no further
action” decision points within the process.
• Incorporates modeling and other
indirect forms of evaluation into the process.
According to the EPA, this guidance
document is not intended to provide recommendations for
delineating the extent of risk or eliminating risk. It is
suggested for use at RCRA Corrective Action sites, National
Priorities List and Superfund Alternative sites, and
Brownfields sites, but is not recommended for use at Subtitle I
Underground Storage Tank sites at this time. More information
is available online at www.epa.gov/correctiveaction/eis/vapor.htm.
Thomas Dong is director of environmental
services for the Long Beach, CA, office of SCS Engineers. He
has more than 27 years of technical and practical experience in
environmental engineering, environmental chemistry, hazardous
waste characterization and management, and project management.
SCS Engineers’ Southern California operation provides
economical environmental solutions to both public and private
sector clients throughout California and the U.S. SCS Engineers
offers specialized expertise in solid waste and recycling
facility design, landfill gas collection and energy recovery,
landfill engineering, Phase I and Phase II site assessments,
hazardous materials management, regulatory compliance,
Brownfields consulting and remediation, asbestos abatement,
storm water compliance, methane mitigation, and storage tank
services. The Southern California headquarters is located at
3711 Long Beach Boulevard, Ninth Floor, Long Beach, California
90807.
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