Formatted Title
Development and Application of Attenuation Factors: Evaluating Migration of Vapor-Forming Chemicals from Subsurface to Indoor Air in Commercial, Industrial, and Other Nonresidential Buildings
Background/Objectives
Groundwater (GW)- and subslab soil gas (SSSG)-to-indoor air (IA) attenuation factors (AFs) (also referred to as GW and SSSG AFs) are frequently used as key components of vapor intrusion (VI) pathway assessments and VI screening level (VISL) development for GW and SSSG. The United States (U.S.) Environmental Protection Agency (EPA) developed a default GW AF of 10E-3 (0.001) and a default SSSG AF of 0.03 based on volatile organic compound (VOC) data in their 2012 VI database that were collected in residential buildings. EPA’s 2015 final VI guidance acknowledges that these default AFs may be overly conservative for nonresidential buildings. Further, the guidance encourages the use of site-specific data and multiple lines of evidence when assessing VI potential. The objectives of this presentation are to demonstrate that VISLs derived from the EPA default GW and SSSG AFs based on residential empirical data are overestimating VI potential in large commercial and industrial buildings using the information from various Department of Defense (DoD) installations and that site-specific data from commercial/industrial buildings support the use of alternative default AFs when assessing VI in nonresidential buildings.
Approach/Activities
Beginning in 2015, the DoD developed and later expanded upon in 2019 a commercial/industrial building VI database larger in size than EPA’s database used to calculate the residential default GW and SSSG AFs. The DoD database consists of VI data collected at DoD installations across the U.S. to assess AFs at commercial and industrial buildings to identify representative alternative default GW and SSSG AFs. Empirical GW and SSSG AFs were calculated from the DoD VI database using the same screening methods employed by EPA that included filtering out potential background source contributions unrelated to VI. The DoD VI database incorporates information that goes beyond IA and subsurface source concentration data, including key information pertaining to exterior and interior building construction, sampling zones, hydrogeology, and distance to the point of primary release of VOCs. Each building in the DoD VI database had one or more sampling zones, defined as an enclosed, occupied location within a building with limited air mixing between other zones and where at least one IA sample was collected. The DoD VI database consists of 76 buildings with one or more sampling zones, for a total of 274 sampling zones evaluated. IA-GW and IA-SSSG concentration data pairs were generated at the sampling zone level on a sampling event basis. This means that whenever IA and SSSG data were available for a given sampling event in a given sampling zone, one IA concentration and one SSSG concentration representative of the sampling event and sampling zone were paired and used for the respective AF statistical analysis.
Results/Lessons Learned
Analysis of IA-GW and IA-SSSG data pairs found that there is significantly more attenuation occurring from GW and SSSG to indoor air in DoD commercial and industrial buildings relative to residential buildings. The results of these analyses are summarized in two peer-reviewed publications (Hallberg et al., 2021; Levy et al., 2023). The DoD commercial/industrial building AFs are one-to-four orders of magnitude lower than EPA’s residential default AFs. The results of the analyses conducted on the DoD VI database of commercial and industrial buildings support the use of a default (95th percentile) GW AF of 10E-4 (0.0001) and SSSG AF of 10E-3 (0.001) for making conservative IA concentration predictions based on GW and SSSG data and deriving VISLs for assessing the VI pathway. These DoD default GW and SSSG AFs have been incorporated into two finalized factsheets as supplements to DoD’s (2009) VI Handbook and will soon be posted on DoD’s DENIX website. The DoD VI database results are consistent with recommendations made by select states to use less conservative AFs for nonresidential structures, including Wisconsin, North Carolina, Indiana, Pennsylvania, and Georgia. The DoD default AFs for industrial and commercial buildings derived using the DoD VI database will result in accelerated evaluation and reduced costs associated with assessing the VI pathway at similar buildings.