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

Abstract

We report a field study of soil gas transport of volatile organic compounds (VOCs) into a slab-on-grade building found at a site contaminated with gasoline. Although the high VOC concentrations (30-60 g nr 3 ) measured in the soil gas at depths of 0.7 m below the building suggest a potential for high levels of indoor VOC, the measured indoor air concentrations were lower than those in the soil gas by approximately six orders of magnitude (~ 0.03 mg nr 3 ). This large ratio is explained by 1) the expected dilution of soil gas entering the building via ambient building ventilation (a factor of ~ 1000), and 2) an unexpectedly sharp gradient in soil gas VOC concentration between the depths of 0.1 and 0.7 m (a factor of ~ 1000). Measurements of the soil. physical and biological characteristics indicate that a partial physical barrier to vertical transport in combination with microbial degradation provides a likely explanation for this gradient. These factors are likely to be important to varying degrees at other sites.

An Evaluation of Vapor Intrusion Into Buildings through a Study of Field Data

“…in their investigation and evaluation of indoor-air impacts at a gasoline contaminated site in California. 6 Additional lines of evidence of this mechanism observed during the course of this study are presented and discussed below.…”

Indoor Air Pollutants and the Impact on Human Health

“…There are, however, some specific outdoor sources that can lead to higher concentrations of certain VOCs indoors than in the general outdoor air environment. For example, gases generated in landfills or from petroleum contamination can migrate through the soil and groundwater to adjacent buildings and homes to give larger indoor concentrations, particularly in basements and crawl spaces, than otherwise expected (Moseley & Meyer, 1992;Hodgson et al, 1992;Fischer et al, 1996). In one such case, the total hydrocarbon concentration was measured to be 120 ppm in a crawl space beneath the floor of a school where petroleum contamination was present from adjacent sources, compared to < 80 ppb outdoors (Moseley & Meyer, 1992).…”

Radon entry into buildings: Effects of atmospheric pressure fluctuations and building structural factors

“…First, the results of this dissertation are directly applicable to the transport of other gas-phase soil contaminants into houses. Recent theoretical and experimental (Wood and Porter, 1987;Fischer et al, 1995) studies suggest that contaminated soil can be an important source of indoor volatile contaminants. Although little research has been done to determine the magnitude of this source, the understanding of the processes affecting radon entry into houses can be used as a basis for understanding the transport of volatile contaminants into houses and for estimating the associated health risk.…”