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

Abstract

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.