How many samples and when to sample? Using new methods and technologies to improve the understanding of our sites
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Subslab soil vapour and indoor air sampling and analysis are common lines of evidence for assessing potential risks to human health associated with subsurface vapour intrusion to indoor air for volatile organic compounds (VOCs).

Conventional subslab soil vapour sampling has shown substantial spatial variability. Indoor air sampling has shown substantial temporal variability and is also susceptible to false positives from interior sources. This often makes data interpretation difficult. Through funding by the United States Department of Defense, new methods and technologies have been developed that reduce the variability in vapour intrusion assessment, especially in large buildings. Two main tools were used to improve our understanding of the vapour intrusion conceptual site model: high volume sampling (HVS) and building pressure cycling (BPC). These will be discussed along with case studies to demonstrate these new technologies.

HVS is a new method of monitoring subslab soil vapour over a large area and extracts a large volume of soil vapour from beneath the floor slab of a building. Laboratory samples are taken at predetermined intervals for speciation and regular field screening is conducted to assess the trend of VOC concentrations as a function of the volume removed. This method provides insight into the spatial distribution of vapours at progressive distances away from the point of extraction. HVS sampling can also provide all the design data needed for mitigation system design at the investigative stage for minimal additional cost. This approach not only minimizes the risk of failing to identify the areas of elevated soil vapour concentrations that may exist between discrete sample locations but does it with fewer holes drilled through the floor and fewer samples for analysis. This results in a higher level of certainty with comparable or lower costs compared to conventional methods.

BPC is a technique for quickly characterizing building susceptibility to vapour intrusion, determining interior sources and estimating the potential vapour intrusion risks. BPC can control the pressure inside a building by using a blower door or existing air handing units to depressurize or pressurize the building. Under these conditions, vapour intrusion can be enhanced or inhibited. Indoor air samples are collected during each of these steps for laboratory analysis to evaluate if vapour intrusion poses a potential concern. BPC tests can be conducted in one day for most buildings and can provide data that characterizes the range of potential impacts that may arise from vapour intrusion and background sources under the typical operating conditions. By controlling the building to represent seasonal/operational changes it provides a higher level of certainty that you did not sample on the "wrong" day and eliminates the need for seasonal sampling. This results in lower costs while providing data to rapidly evaluate sites for vapour intrusion and strengthen risk management decisions.

Publication Summary

  • Geosyntec Authors: Darius Mali
  • All Authors: Darius Mali
  • Title: RPIC
  • Event or Publication: Event
  • Practice Areas: Contaminated Site Assessment and Cleanup
  • Citation: Geosyntec, Savron (a Geosyntec company), and SiREM (a Geosyntec company practitioners will present virtually at the Workshop from November 15-18, 2021.
  • Date: November 15-18, 2021
  • Location: Online
  • Publication Type: Platform Presentation