Petroleum hydrocarbon vapors biodegrade aerobically in the subsurface. Depth profiles of petroleum hydrocarbon vapor and oxygen concentrations from seven locations in sandy and clay soils across four states of Australia are summarized. The data are evaluated to support a simple model of biodegradation that can be used to assess hydrocarbon vapors migrating toward built environments. Multilevel samplers and probes that allow near-continuous monitoring of oxygen and total volatile organic compounds (VOCs) were used to determine concentration depth profiles and changes over time. Collation of all data across all sites showed distinct separation of oxygen from hydrocarbon vapors, and that most oxygen and hydrocarbon concentration profiles were linear or near linear with depth. The low detection limit on the oxygen probe data and because it is an in situ measurement strengthened the case that little or no overlapping of oxygen and hydrocarbon vapor concentration profiles occurred, and that indeed oxygen and hydrocarbon vapors were largely only coincident near the location where they both decreased to zero. First-order biodegradation rates determined from all depth profiles were generally lower than other published rates. With lower biodegradation rates, the overlapping of depth profiles might be expected, and yet such overlapping was not observed. A model of rapid (instantaneous) reaction of oxygen and hydrocarbon vapors compared to diffusive transport processes is shown to explain the important aspects of the 13 depth profiles. The model is simply based on the ratio of diffusion coefficients of oxygen and hydrocarbon vapors, the ratio of the maximum concentrations of oxygen and hydrocarbon vapors, the depth to the maximum hydrocarbon source concentration, and the stoichiometry coefficient. Whilst simple, the model offers the potential to incorporate aerobic biodegradation into an oxygen-limited flux-reduction approach for vapor intrusion assessments of petroleum hydrocarbon compounds.

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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.

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The release and retention of in-situ colloids in aquifers play an important role in the sustainable operation of managed aquifer recharge (MAR) schemes. The processes of colloid release, retention, and associated permeability changes in consolidated aquifer sediments were studied by displacing native groundwater with reverse osmosis-treated (RO) water at various flow velocities. Significant amounts of colloid release occurred when: (i) the native groundwater was displaced by RO-water with a low ionic strength (IS), and (ii) the flow velocity was increased in a stepwise manner. The amount of colloid release and associated permeability reduction upon RO-water injection depended on the initial clay content of the core. The concentration of released colloids was relatively low and the permeability reduction was negligible for the core sample with a low clay content of about 1.3%. In contrast, core samples with about 6 and 7.5% clay content exhibited: (i) close to two orders of magnitude increase in effluent colloid concentration and (ii) more than 65% permeability reduction. Incremental improvement in the core permeability was achieved when the flow velocity increased, whereas a short flow interruption provided a considerable increase in the core permeability. This dependence of colloid release and permeability changes on flow velocity and colloid concentration was consistent with colloid retention and release at pore constrictions due to the mechanism of hydrodynamic bridging. A mathematical model was formulated to describe the processes of colloid release, transport, retention at pore constrictions, and subsequent permeability changes. Our experimental and modeling results indicated that only a small fraction of the in-situ colloids was released for any given change in the IS or flow velocity. Comparison of the fitted and experimentally measured effluent colloid concentrations and associated changes in the core permeability showed good agreement, indicating that the essential physics were accurately captured by the model.

Potential hydrocarbon-vapor intrusion pathways into a building through a concrete slab-on-ground were investigated and quantified under a variety of environmental conditions to elucidate the potential mechanisms for indoor air contamination. Vapor discharge from the uncovered open ground soil adjacent to the building and subsequent advection into the building was unlikely due to the low soil-gas concentrations at the edge of the building as a result of aerobic biodegradation of hydrocarbon vapors. When the building's interior was under ambient pressure, a flux of vapors into the building due to molecular diffusion of vapors through the building's concrete slab (cyclohexane 11 and methylcyclohexane 31 mg m(-2) concrete slab day(-1)) and short-term (up to 8 h) cyclical pressure-driven advection of vapors through an artificial crack (cyclohexane 4.2 x 10(3) and methylcyclohexane 1.2 x 10(4) mg m(-2) cracks day(-1)) was observed. The average subslab vapor concentration under the center of the building was 25,000 microg L(-1). Based on the measured building's interiorvapor concentrations and the building's air exchange rate of 0.66 h(-1), diffusion of vapors through the concrete slab was the dominantvapor intrusion pathway and cyclical pressure exchanges resulted in a near zero advective flux. When the building's interior was under a reduced pressure (-12 Pa), advective transport through cracks or gaps in the concrete slab (cyclohexane 340 and methylcyclohexane 1100 mg m(-2) cracks day(-1)) was the dominant vapor intrusion pathway.

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