1009679 - More Efficient and Defensible Sampling Strategies to Assess Vapor Intrusion Temporal and Spatial Variability in a Complex Industrial Building

Limited information is available on the degree and causes of spatial and temporal variability of indoor air concentrations in industrial buildings subject to vapor intrusion (VI). An extensive data set was collected under natural conditions in five sampling zones within a large industrial World War II-era building. Data were collected continuously for 12 months, including TCE, radon, differential pressure, barometric pressure, differential temperature, wind speed and precipitation. The objectives for this work included: 1) assessing VOC temporal variability in an industrial building; 2) evaluating whether the use of VI indicators/tracers, which are less costly to measure, may be able to estimate VOC concentrations or predict trends; and 3) define strategies and conditions for identifying and selecting industrial/commercial building sampling zones and time of sampling for optimizing VI evaluations. This work was funded under Navy Environmental Sustainability Development to Integration (NESDI) Project 554.
Indoor air concentrations as an annual average varied spatially by up to a twenty-fold factor between sampling zones. Indoor air concentrations varied temporally by up to approximately two orders of magnitude (expressed as 24-hour rolling average concentrations) in the two zones within the building where VI was occurring. Indoor radon concentrations measured with an inexpensive consumer grade monitor were well correlated with TCE in one of the zones where VI was occurring, but not the other (potentially due to differing air exchange rates). The TCE and radon data collected generally reflect greater concentrations during period of highest indoor/outdoor differential temperature (DT). This pattern likely is the result of stack effects and associated increased soil gas entry during the heating season. The data, however, show that the most elevated TCE or radon concentrations do not coincide with the most elevated DT, potentially because increased DT also induces increased air exchange.
The indoor air data obtained as part of the program generally support defining a sampling zone as a group of abutting rooms that share a common HVAC system from which only one indoor air sample paired with a subslab soil gas sample may be needed. However, additional indoor air samples should be considered in zone subareas (e.g., room or rooms within the zone) where vapor entry points may be present and the zone subarea is enclosed (e.g., door most often closed), such that air mixing is not occurring with the rest of the zone. Based on the experience in this building, a strategy will be presented for combining visual building survey observations with indicator/tracer data to produce an efficient sampling strategy and increase the defensibility of VI assessments.