HAEP Release-Response Working Group Presentation - June 28, 2017
Use of High-Resolution Passive Sampler Data and Large Volume Purge Subslab Vapor Data to Improve Assessment of Vapor Intrusion Risk
Presented by Dr. Roger Brewer, State of Hawaii Department of Health
Date: Wednesday, June 28, 2017
Time: Noon.
Location: HECO Conference Room ASB2, 8th Floor, American Savings Bank Tower, 1001 Bishop Street
Synopsis:
Representative subslab vapor data are important to tie VOCs identified in indoor air to subsurface contamination. The use of traditional, small-volume, “discrete” vapor samples to characterize vapor plumes and assess vapor intrusion risk is unreliable, however. In this study, a combination of passive sampler data and Large Volume Purge (LVP), active sample data were used to characterize and assess a previously identified, PCE vapor plume beneath a former dry cleaner. The slab was divided into twenty-five, 400 ft2 cells. Four passive samplers were installed in each cell and combined for analysis in order to better capture and represent random, small-scale variability of VOCs with a cell. Triplicate sets of samplers were installed in three cells in order to test the field precision of the data (average RSD 14%). The resulting data indicate a much larger plume than previously identified. Concentration trends between a subset of samplers that were individually tested were random and non-linear. This implies that concentration patterns generated around individual data points by isoconcentration mapping programs could be artificial. A series of five, 7,000 liter LVPs was then carried out from a hypothetical, vapor entry point installed in the center of the slab. Each LVP represents USEPA’s default “Decision Unit” volume of vapors assumed to intrude into a building in a single day (vapor entry rate 5 L/minute). A representative sample of each LVP was collected by continuously drawing vapors from the purge stream into a six-liter summa canister. The concentration of PCE in the LVP samples increased with increasing purge volumes. This suggests that outlying, isolated “hot spot” areas of the plume were progressively captured and incorporated into the samples with increasing purge volume. The results provide a much better resolution of the subslab vapor plume and more reliable data for assessment of vapor intrusion risk.
This talk is part of the Hawaii Department of Health/HAEP Release-Response Working Group, which is committed to the increased dissemination of technical knowledge to the Hawaii Environmental Professional Community. As such, this talk is presented free of charge. Brown-bag lunch and drinks are permitted in the room during the talk. Thanks to HECO for providing the meeting space.
Representative subslab vapor data are important to tie VOCs identified in indoor air to subsurface contamination. The use of traditional, small-volume, “discrete” vapor samples to characterize vapor plumes and assess vapor intrusion risk is unreliable, however. In this study, a combination of passive sampler data and Large Volume Purge (LVP), active sample data were used to characterize and assess a previously identified, PCE vapor plume beneath a former dry cleaner. The slab was divided into twenty-five, 400 ft2 cells. Four passive samplers were installed in each cell and combined for analysis in order to better capture and represent random, small-scale variability of VOCs with a cell. Triplicate sets of samplers were installed in three cells in order to test the field precision of the data (average RSD 14%). The resulting data indicate a much larger plume than previously identified. Concentration trends between a subset of samplers that were individually tested were random and non-linear. This implies that concentration patterns generated around individual data points by isoconcentration mapping programs could be artificial. A series of five, 7,000 liter LVPs was then carried out from a hypothetical, vapor entry point installed in the center of the slab. Each LVP represents USEPA’s default “Decision Unit” volume of vapors assumed to intrude into a building in a single day (vapor entry rate 5 L/minute). A representative sample of each LVP was collected by continuously drawing vapors from the purge stream into a six-liter summa canister. The concentration of PCE in the LVP samples increased with increasing purge volumes. This suggests that outlying, isolated “hot spot” areas of the plume were progressively captured and incorporated into the samples with increasing purge volume. The results provide a much better resolution of the subslab vapor plume and more reliable data for assessment of vapor intrusion risk.
This talk is part of the Hawaii Department of Health/HAEP Release-Response Working Group, which is committed to the increased dissemination of technical knowledge to the Hawaii Environmental Professional Community. As such, this talk is presented free of charge. Brown-bag lunch and drinks are permitted in the room during the talk. Thanks to HECO for providing the meeting space.
