Improved vapor intrusion field investigation methods are needed to support cost-effective evaluation of the vapor intrusion pathway. Intensively monitored sites, such as the Borden Landfill in Canada, have greatly contributed to an understanding of the physical and chemical processes that control the transport of chemicals in groundwater. For this project, a similar approach employing intensively monitored sites with specially designed monitoring networks was used to address the critical groundwater-to-indoor-air vapor intrusion pathway.

The development of refined vapor intrusion guidance, step-wise screening, and cost-effective field investigation approaches will benefit facility managers by providing investigation results that support a defensible evaluation of vapor intrusion. Determination of the presence or absence of vapor intrusion impacts is important to the site management process. Definitive determination of the absence of vapor intrusion allows resources to be directed to other site impacts while avoiding presumptive mitigation, which can be burdensome from both financial and public relations perspectives.

The objective of this project was to identify a cost-effective and accurate protocol for investigating vapor intrusion into buildings overlying contaminated groundwater. Three performance goals were established, and all objectives were met, namely,

• Collection of data representative of site conditions
• Determination of vapor intrusion impacts at demonstration sites (i.e., indoor air concentration of chemical above risk-based screening limit, not attributable to background indoor air sources)
• Development of a reliable vapor intrusion investigation approach (i.e., identify a limited scope investigation approach with higher accuracy than current approaches).

The demonstration results supported the use of a step-wise process for evaluating vapor intrusion from groundwater sources. This recommended evaluation process has been documented in a White Paper. The recommended sampling program when evaluation of individual buildings is required is summarized below.

Notes:

1) TO-15 selective ion monitoring (SIM) may be required for indoor and ambient air samples to achieve detection limits below regulatory screening values. TO-15 analyses are conducted by numerous commercial laboratories. The TO-15 analyte list may vary between laboratories and should be reviewed to ensure inclusion of all volatile chemicals of concern (COC). 2) Radon samples analyzed by Dr. Doug Hammond (dhammond@usc.edu) at the University of Southern California Department of Earth Sciences as described in McHugh et al. (2008). Analysis of radon in gas samples is not currently available from commercial environmental laboratories; however, Dr. Hammond will conduct the analysis for environmental consultants and other parties. 3) Recommended number of samples for a typical residence with a 1,000-2,000 ft² foundation. Additional samples may be appropriate for larger structures.

Spatial and temporal variability in volatile organic compound (VOC) concentrations has a significant impact on vapor intrusion investigations. High spatial and long-term temporal variability in soil gas (SG) VOC concentration results in high uncertainty associated with VOC transport through the vadose zone. Because of this high variability, a large number of sample locations and sampling events is needed to accurately characterize the VOC distribution in soil gas.

Other observations and lessons learned concern the sampling and analysis process. Summa canisters are the most commonly used containers for SG or air sample collection. Because these canisters are typically provided by the laboratory and reused many times, care must be taken to prevent cross-contamination between sample events. Another important consideration with SG samples is the use of a leak tracer, important to ensure that the collected sample is not impacted by significant leakage of ambient air. Some leak tracer compounds such as difluoroethane (DFA) and isopropyl alcohol may cause elevated detection limits for target compounds. It is important to confirm with the analytical laboratory that the tracer compound will not interfere with the analysis of target compounds.

The demonstration results have been used to develop a recommended approach for cost-effective, building-specific evaluation of vapor intrusion impacts at corrective action sites. It is important to note, however, that the understanding of vapor intrusion is evolving rapidly and that the recommended approach may not satisfy all regulatory requirements. The end user should review applicable guidance and regulations and modify or supplement this approach to ensure that regulatory requirements are satisfied. (Project Completed - 2008)