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Measurement and Modeling of Temporal Variations in Hydrocarbon Vapor Behavior in a Layered Soil Profile

CSIRO Land and Water, Private Bag 5, Wembley, W.A., 6913, Australia

Correspondence: ^* Corresponding author (Greg.Davis{at}csiro.au)

Received for publication 29 January 2004. Fine-scale measurement of gasoline vapors, major gases (O₂, CO₂, N₂, and CH₄), residual nonaqueous phase liquid (NAPL) gasoline, and soil physical properties has allowed detailed assessment of the role of soil layering and seasonal variability on hydrocarbon vapor fate and biodegradation. In this study we conducted coring and static depth profile monitoring at the end of summer and end of winter for a layered sandy vadose zone in Perth, Western Australia. Transient on-line monitoring of vapors and O₂ was also performed with in situ multilevel volatile organic compound (VOC) and O₂ probes. For high soil moisture contents at the end of winter, vapors were shown to accumulate beneath a compacted, cemented layer approximately 0.3 m below the ground surface, and O₂ penetrated only to depths of 0.4 m below ground. At the end of summer, when soil moisture was lower, O₂ penetrated to depths of up to 1.5 m, and hydrocarbon vapors remained at or below this depth. Regardless of seasonal changes, sharp separations were seen between the depth of O₂ penetration from the ground surface and the depth of penetration of the vapors upward from the hydrocarbon-contaminated zone. Modeling of steady-state O₂ profiles indicated that a number of simple O₂ consumption models might apply, including point-sink, distributed zero-order, or distributed first-order models, each leading to different biodegradation rates. Combining independent data with modeling helped determine the most appropriate model, and hence estimates of O₂ consumption and hydrocarbon biodegradation. Also, reliable estimates of the biodegradation rate could only be calculated after consideration of the layered features.

Abbreviations: BTEX, benzene, toluene, ethylbenzene and the isomers of xylene • NAPL, nonaqueous phase liquid • NOM, natural organic matter • TOC, total organic C • TPH, total petroleum hydrocarbons • VOC, volatile organic compound

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