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Vadose Zone Natural Attenuation Of Hydrocarbon Vapors

An Empirical Assessment Of Soil Gas Vertical Profile Data

API Soil and Groundwater Research Bulletin Number 15
December 2001

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This report describes an empirical assessment of available soil gas profile data with an emphasis on petroleum hydrocarbon release sites. The objectives included the following: i) identify data sets with sufficient detail (i.e., hydrocarbon and oxygen soil gas profiles and lithologic information at a minimum), ii) review the data and identify the range of behaviors observed, iii) assess whether or not the data was consistent with the occurrence of aerobic biodegradation, and iv) assess the significance of aerobic biodegradation on soil gas profiles and hydrocarbon fluxes.

This was accomplished by: a) compiling a database of published and unpublished data from hydrocarbon spill sites where soil gas profiles had been monitored, b) collecting field data at three additional sites, and then c) conducting a data analysis. At most sites, moisture content, effective diffusion coefficient, subsurface characterization data, and vapor concentration profiles of oxygen, carbon dioxide, and petroleum hydrocarbon were measured. Complete details of the site characteristics, and data compilation and initial analysis can be found in Roggemans (1998).

In brief, four general categories of soil gas profiles were identified. In each case the data supported the hypothesis that aerobic biodegradation was an important factor in the development of the measured soil gas profiles; however, the significance of biodegradation to ongoing vapor transport varied across the categories.