In 1993, as a result of corporate safety, health and environmental policies, the client initiated a review to assess potential hazards to human health and the environment resulting from historical industrial waste disposal activities.
Geosyntec's Scope of Services for Vapour Intrusion Assessment
In 1998, the potential for vadose zone vapour transport of chlorinated hydrocarbon compounds (CHCs) towards adjacent housing was identified as the most significant potential hazard needing further evaluation. A proximal vapour monitoring well (VMW) network was installed around the perimeter of the disposal site in 1998, and the data from these VMWs confirmed a potential concern around CHCs. Indoor air quality (IAQ) monitoring was therefore conducted in parallel with the installation of a second phase of distal VMWs. Indoor air and subsurface soil gas monitoring, as well as geological investigations and historical review, identified the houses with elevated indoor air levels of CHCs as located on highly permeable backfill material, while houses built on bedrock did not have elevated indoor air levels of CHCs. This discovery resulted in the demolition of some houses.
The potential indoor air levels of CHCs have been successfully predicted using subsurface concentrations, which are the source of potential vapour inhalation risks, and specific subsurface to indoor air attenuation coefficients have been calculated using a tracer compound. The predicted levels were confirmed by indoor air monitoring of CHCs in a subset of the houses in the indoor air monitoring study group. Subsurface vapour concentrations can, therefore, be used to evaluate potential risks from inhalation of indoor air, and are monitored on an on-going semi-annual basis.
Other pathways of contaminant transport from the disposal site to potential receptors have been evaluated using both site- specific data directly as well as various models as part of a comprehensive risk assessment. These pathways include plant uptake, potential impacts to drinking water and exposure via soils.
Notable Accomplishments of Vapour Intrusion Assessment
Identification of CHCs as compounds of potential significance, and elucidation of the mechanisms through which CHCs are transported from an industrial waste disposal site to indoor air has allowed specific remedial activities to be implemented. Well- executed data collection and analysis has been shown to provide the detailed conceptual understanding of site conditions required to make decisions regarding site management.