Schematic of soil-gas and contaminant transport into a slab-on-grade building at a former service station site. Three effects are illustrated that can contribute to reducing the amount of contaminant available for entry into the building: biodegradation by soil microorganisms; a layer of soil that limits diffusive movement of the contaminant; and wind-driven ventilation of the soil below the building. Not illustrated are the effects of ventilation on contaminant concentrations inside the building.
The transport of soil-gas-borne contaminants into buildings has been documented as a significant source of human exposure to some pollutants indoors; one example is radon, which has received widespread public attention. Other gas-phase chemicals, such as volatile organic compounds, may show similar behavior, although these have received less scientific and public attention. Leaky underground storage tanks for gasoline and other petroleum hydrocarbons are a cause of concern for several reasons. Exposure to some of the compounds in these mixtures has known human health implications, and studies have reported indoor air contamination by some of these species. Also, a large number of storage tanks--some in urban areas--are leaking. Estimating indoor air VOC concentrations caused by subsurface sources is an important input to assessments of health effects and assignments of priorities for remediation activities. However, because such estimates of exposure usually depend on the sophistication of the models and their assumptions, the results often vary by orders of magnitude.
In an effort to understand the factors that affect soil gas contaminant transport into buildings, we conducted a field study at a former gasoline service station building at the Alameda Naval Air Station (ANAS), in California. This station was closed in the late 1980s following gasoline leaks from a damaged underground storage tank and from feed lines to the dispensing pumps. We began by measuring the VOC concentrations in the outdoor and indoor air, soil gas, and groundwater. Although high concentrations (~30 g m^[-3]) of several compounds found in gasoline were measured in the soil gas 0.7 m (2.3 ft) below the building, the measured indoor air concentrations in the building were approximately six orders of magnitude lower, a much larger difference than the three orders of magnitude more typically observed. Our study then focused on factors that might limit or influence transport, and on the potential for biological degradation of the VOCs. Measurements included depth profiles of selected VOCs, CH4, O2, and CO2; the chemical and physical properties of the soil; tracer-gas tests of diffusive and advective transport in the soil and of the buildings ventilation characteristics; and a controlled test of the rate of biodegradation of selected VOCs in the soil.
The soil-gas concentration profile of several chemical species in the soil below the building showed a sharp increase in the isopentane (a volatile constituent of gasoline) and methane concentrations between depths of 0.4 and 0.65 m (1.3 and 2.1 ft) compared with concentrations at shallower depths. At the same time, the concentration of CO2 also increased somewhat while that of O2 decreased, again with the largest changes observed at depths between 0.45 and 0.6 m. Because these concentration profiles are suggestive of aerobic consumption of the hydrocarbons by soil microorganisms, we performed laboratory incubation experiments using soils collected from the site at various depth intervals. The biodegradation rates observed in the laboratory experiments were somewhat greater than those observed in the field but appear to be consistent when accounting for the differences between the conditions of the lab and field experiments are taken into account.
We combined these results in the context of a simplified schematic model, illustrated in the figure, to estimate the effects of building ventilation, physical limitations to soil-gas transport, and biodegradation on indoor contaminant concentrations. As the figure suggests, the combination of biodegradation and restricted transport in the low-diffusivity layer below the building alters the amount of contaminant in the soil gas available to be transported into the building. We estimate that dilution of soil gas entering the building via ambient ventilation reduces the VOC concentration by a factor of ~1,000, that the physical limitations to transport reduce the soil-gas concentrations of all gas species by a factor of ~10, and that biodegradation reduces the concentration of these VOCs by another factor of ~100.
These physical and biological processes are likely to affect indoor air concentrations of contaminants to varying degrees at other sites. Although aliphatic petroleum hydrocarbons may undergo near-surface aerobic biodegradation faster than other compounds in less aerobic environments (particularly halogenated hydrocarbons in the deep subsurface), these and other types of biodegradation may have a substantial effect on the indoor air concentrations observed at a site. Our results--the unanticipated low levels of VOCs in the ANAS facility--suggest that estimating VOC transport into buildings in a site or risk assessment requires careful attention to identifying and separating physical and biotic effects.
--Richard Sextro and Marc Fisher
Richard Sextro
Indoor Environment Program
(510) 486- 6295; (510) 486-6658 fax
This research is sponsored by the National Institute of Environmental Health and Safety and DOE's Office of Energy Research, Office of Health and Environmental Research.