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In recent years, the topic of human respiratory exposures from gases and vapors coming from contaminated sites has gained interest. The atmosphere in the unsaturated zone gets contaminated by volatile organic compounds when petroleum or chlorinated liquids are released into the soil. As the liquid is introduced, most of compounds enter into different phases and establish a chemical equilibrium. This equilibrium is affected by varying factors, such as moisture content, barometric pressure, temperature and microbial activity. For this reason, gas and vapor concentrations in the vadose zone vary constantly. To achieve accurate soil atmosphere sampling, many basic concepts must be considered. Among these, the collection of soil atmosphere samples at the lowest flowrate possible and the analysis of gases and vapors at each sampling location are of primary importance. Nevertheless even if active sampling is performed in the best conditions, the dynamic nature of the soil atmosphere will always remain a challenge for the assessment of long term conditions. This paper describes the induced flux method, an innovative sampling technique which uses clean nitrogen to flush the subsurface during a short period of time. This flush alters the “soil-liquid-gas” system equilibrium and causes free liquid, sorbed and dissolved compounds of concern to transfer to the gas phase. When the equilibrium is clearly unbalanced, the rate at which the gas or vapor are transferred to the soil atmosphere is evaluated and induced flux data are calculated. This data may be collected on site with direct reading instruments within 20 minutes per sampling location. Concentrations coupled with induced flux data provide details on potential atmospheric intrusion and on migration of soil gas and vapor around the sampling location. The author suggests different approaches to assess long term conditions of the soil atmosphere when both types of data are available at a site.
soil gas, vadose zone, contaminant, volatile organic compound, sampling, environmental monitoring, vapor intrusion, petroleum hydrocarbons, chlorinated solvents
WESA, a division of Blumetric Environmental Inc., Montreal,