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

Abstract

Jury et al.Fine-scale measurement of gasoline vapors, major gases (O 2 , CO 2 , and Baehr, 1996; Anderssen et al., 1997; Anderssen and N 2 , and CH 4 ), residual nonaqueous phase liquid (NAPL) gasoline, Markey, 1997; Bekins et al., 1998;Johnson et al., 1999; and soil physical properties has allowed detailed assessment of the role of soil layering and seasonal variability on hydrocarbon vapor Turczynowicz and Robinson, fate and biodegradation. In this study we conducted coring and static 2001), some in combination with field studies (Barber depth profile monitoring at the end of summer and end of winter for and Davis, 1991aand Davis, , 1991bÖ hman, 1999;Hers et al., 2000). a layered sandy vadose zone in Perth, Western Australia. Transient Despite the modeling efforts and related work, there on-line monitoring of vapors and O 2 was also performed with in situ are still only limited field data sets with sufficient detail multilevel volatile organic compound (VOC) and O 2 probes. For high for evaluating vapor processes in impacted soil profiles soil moisture contents at the end of winter, vapors were shown to and for model validation. Additional well-documented accumulate beneath a compacted, cemented layer approximately 0.3 m studies are required (Johnson et al., 1999). Also, changes below the ground surface, and O 2 penetrated only to depths of 0.4 m in soil moisture distribution and soil layering have been below ground. At the end of summer, when soil moisture was lower, reported to impact vapor behavior and lead to complica-O 2 penetrated to depths of up to 1.5 m, and hydrocarbon vapors remained at or below this depth. Regardless of seasonal changes, sharp tions when estimating biodegradation rates (Johnson separations were seen between the depth of O 2 penetration from the and Perrott, 1991; Fischer et al., 1996;. ground surface and the depth of penetration of the vapors upwardWe present the results of field research and modelfrom the hydrocarbon-contaminated zone. Modeling of steady-state ing to quantify the role of a fine-scale moisture-retentive O 2 profiles indicated that a number of simple O 2 consumption models layer in a soil profile in changing the subsurface distribumight apply, including point-sink, distributed zero-order, or distribtion of gasoline vapors and the major gases due to seauted first-order models, each leading to different biodegradation rates. sonal changes in moisture contents. Simple analytical Combining independent data with modeling helped determine the and numerical modeling was performed to assess the most appropriate model, and hence estimates of O 2 consumption and impact of moisture variability on estimates of the biohydrocarbon biodegradation. Also, reliable estimates of the biodegdegradation rate based on depth profiles. Coring, depth radation rate could only be calculated after consideration of the layered features.

Analytical modelling of stable isotope fractionation of volatile organic compounds in the unsaturated zone

“…In the unsaturated zone, biodegradation of VOC occurs only in the aqueous phase and can be described by a variety of kinetic models (zero order, first order, Monod kinetics, or instantaneous bimolecular reaction). The last two apply to conditions of low oxygen partial pressure and reaction fronts of hydrocarbons with oxygen (Davis et al, 2005(Davis et al, , 2009). For fully aerobic conditions, Hohener et al (2003) explain how the kinetics of biodegradation is related to the VOC concentration in the gas phase, and how Henry's law is involved therein.…”

Experimental study on retardation of a heavy NAPL vapor in partially saturated porous media

“…Nonaqueous-phase liquid (NAPL) contaminants introduced into the unsaturated zone spread as a liquid phase; however, they can also vaporize and migrate in a gaseous state. In particular, vapor (gas) plumes migrate easily in the unsaturated zone (Barber and Davis, 1991;Davis et al, 2005Davis et al, , 2009Höhener et al, 2006;Brusseau et al, 2013). When assessing the danger of groundwater contamination by downwardmigrating vapor plumes, retention effects on transport are of major interest.…”

Variability of soil potential for biodegradation of petroleum hydrocarbons in a heterogeneous subsurface

“…At petroleum-contaminated sites in general, two specific limitations of intrinsic biodegradation in the unsaturated zone are reported: (i) availability of oxygen and (ii) availability of macronutrients. The former is related to soil airfilled porosity and distance to the ground level [22][23][24][25][26][27] whilst the latter is mostly related to nitrogen deficiency in contaminated soil zones where the C: N ratio is high [28][29][30].…”