Vapor intrusion occurs when there is a migration of vapor-forming chemicals from any subsurface contaminant source into an overlying building. Vapor intrusion can lead to indoor air quality concerns and introduce contaminants of concern, such as volatile
organic compounds (VOCs), into an indoor environment. Vapor intrusion can represent an immediate pathway for human health concerns due to inhalation. As of December 2018, 42 states have issued draft or final vapor intrusion guidance documents.
Individual state regulatory agencies may also oversee assessment and remediation/mitigation of sites affected by vapor intrusion.
Trihydro has been a leader in vapor intrusion assessment since the early 2000s. Our specialists have carried out vapor intrusion studies in 17 states across the United States and have been involved in developing state and Interstate Technology
and Regulatory Council (ITRC) vapor intrusion guidance documents. Trihydro has given multiple presentations on emerging vapor intrusion issues such as preferential sewer pathways and short-term trichlororethene (TCE) exposure.
We routinely assist municipalities, state and federal agencies, and the private sector with vapor intrusion-related projects. Our vapor intrusion experience includes a range of project sizes, from Phase I Environmental Site Assessment (ESA) vapor
encroachment assessments, small-scale worker safety assessments, and rapid mitigation system installation, to serving as the technical lead for multiple regional vapor plume assessments, including collecting vapor data to support local, neighborhood-wide,
and regional evaluation of residential vapor intrusion risk assessment.
When vapor intrusion is suspected, technicians may collect various samples to assess the presence of VOCs. Soil gas and groundwater samples may be taken from areas around a site and compared to samples taken on-site, such as sub-slab samples or
indoor air samples. This comparison helps identify a chemical's source and determine whether vapors have migrated indoors. However, a building's makeup may contain the same chemicals being assessed, making it difficult to determine the source
of various chemicals. If vapor intrusion is confirmed, a mitigation system can be installed to address concerns.
Trihydro’s common sense approach to vapor intrusion projects involves the application of multiple lines-of-evidence methods to determine the potential sources and account for background sources and to identify potentially complete exposure
pathways and receptors. This information is then distilled into a vapor conceptual site model (CSM), which is used to design a strategy that includes input from multiple stakeholders. If the cost to mitigate the vapor hazard is lower than
the assessment costs, Trihydro works with clients to determine if proactive mitigation is the more appropriate path to closure based on evaluation of the potential liability associated with each option, long-term operation and maintenance
costs, and site access concerns.
In addition to evaluating vapor data as a direct media of concern, Trihydro has worked with clients to integrate routine vapor monitoring to natural source zone depletion (NSZD) assessments at petroleum-impacted sites. Specifically, soil gas data
can be used to characterize the natural attenuation processes in the vadose zone and to quantify the biodegradation rates of a petroleum plume. Evaluating NSZD as part of vapor assessments can provide clients with a remedial alternative in
lieu of active mitigation and/or remediation. At sites where NSZD biodegradation rates are appreciable, these data have been used to establish new remedial system endpoints, track progress towards remedial goals, and alleviate public concerns
regarding contaminant longevity.
Trihydro has decades of experience conducting human health risk assessments and has integrated this expertise into vapor intrusion assessments. Using our risk assessment experience, Trihydro provides an accurate evaluation of actual health risk
from the vapor intrusion pathway. Assessing vapor intrusion from this perspective can be valuable in achieving no further action that is effective and cost-conscious.