Factors Affecting Indoor Air Concentrations of Volatile Organic Compounds at a Site of Subsurface Gasoline Contamination

We report a field study of soil-gas transport of volatile organic compounds (VOCs) into a building at a site contaminated with gasoline. High VOC concentra tions (30−60 g m^-3) were measured in soil gas 0.7 m below the building. Measured indoor air concentra tions were ~10⁶ lower due to a sharp gradient in soil-gas VOC concentrations between 0.1 and 0.7 m (a factor of ~10³) and the dilution of soil gas entering the building by wind-driven building ventilation (a factor of ~10³). Measurements of soil physical and biological characteristics indicate that a partial physical barrier to vertical transport in combination with microbial degradation can explain the gradient. While these factors are likely to be important to varying degrees at other sites contaminated with VOC, we conclude that (1) the results of this study cannot be directly applied to estimate indoor air quality at other sites without the risk of incurring significant errors and instead that (2) future attempts to estimate VOC transport into buildings should be made with careful attention to the identification and separation of physical and biotic effects.