Vapor intrusion refers to the phenomenon in which contaminated soil vapor is created by off gassing of volatile organic compound (VOC) contaminants in the subsurface (either in soil or groundwater or both). The soil vapor, in turn, migrates through the soil column and may enter into overlying structures. LEA's vapor intrusion experts specialize in assisting clients with these environmental issues.
Because it is a dynamic process, our vapor intrusion consultants understand that the degree and severity of vapor intrusion into a home is not constant. Recent research (summarized below) has shown that contaminant concentrations in indoor air can vary from day-to-day and season-to-season by a factor of 1,000 or more. In its 2009 review of the Draft Vapor Intrusion Guidance, EPA articulated the evolving understanding of temporal and spatial variability in vapor intrusion: “In summary, EPA’s observations and experiences have indicated that there is greater complexity in the processes and number of variables that affect the migration and distribution of VOCs, and consequently, the potential for vapor intrusion [is greater] than was generally appreciated” (p. 2, emphasis added).
These findings have important implications for environmental scientists and engineers trying to evaluate the risk of vapor intrusion at a particular site. Of principal importance, our vapor intrusion experts believe that one or two tests at or under a home are not generally sufficient for understanding the true risks. If a phenomenon is highly variable and highly complex, more testing is needed to measure its full range of variability. Conversely, if a phenomenon is totally constant, then less testing is needed because there is no variability to measure. For example, if we collected a single temperature reading of 80° F. in Hawaii, this would be a reasonable approximation of average annual temperature because there is so little seasonal variation in Hawaii. However, if we collected a single temperature reading on a warm summer day in Alaska, it may be an accurate reading of the weather on that day but it would be a mistake to conclude that the weather in Alaska is always warm and sunny. More temperature readings are needed in Alaska in order to measure the extreme seasonal variability: the warm summers and the bitterly cold winters. In the same way, repeated testing for vapor intrusion is needed because the phenomenon is highly variable.
Having provided vapor intrusion consulting services nationwide, we have frequently observed a large degree of temporal variability in vapor intrusion. Academic studies back up our experience: showing that conventional sampling strategies of collecting just one or two samples carries a high probability of “false negatives.” The term “false negative” refers to a situation in which a condition exists (such as elevated levels of TCE in indoor air at a home overlying a groundwater plume) but poor design of the testing methodology fails to detect it. These findings are consistent with the vapor intrusion research reported in Dr. Everett’s recent book about soil gas and worst case risk parameters: Everett and Kram, editors, 2013, Continuous Soil Gas Measurements: Worst Case Risk Parameters, ASTM, Selected Technical Papers [STP] 1570. LEA's vapor intrusion consultants have served clients across California, including Los Angeles, San Francisco and San Diego.
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