Evidence for Instantaneous Oxygen‐Limited Biodegradation of Petroleum Hydrocarbon Vapors in the Subsurface

Abstract

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.

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.…”

Role of natural attenuation in modeling the leaching of contaminants in the risk analysis framework

“…Evidence for vadose zone biodegradation of BTEX has been seen at several field studies reporting typical average values of biodegradation constant rates in the order of 0.01 and 10 d À1 (e.g. see Davis et al, 2009;DeVaull et al, 1997;Hers et al, 2000;Hoener et al, 2003). On the contrary, PAHs are largely sorbed to the soil and are slowly biodegradable.…”

“…Finally it is worth noting that the first-order degradation rates used in the model to best fit the field data are achieved assuming values that are in line with those reported in the literature (e.g. Davis et al, 2009;DeVaull et al, 1997;Brauner et al, 2002;Blum et al, 2009;Bockelmann et al, 2001). Namely, as discussed before, typical average median values of biodegradation constant rates for BTEX are in the order of 0.01 and 10 d À1 whereas for PAHs are in the range of 0.0001e0.01 d À1 .…”

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.…”

“…Corley et al (1996) showed that low concentrations of volatile organic compounds distribute in the bulk phases (air, water, and solid), adsorb to the air-water interface, and partition into intraparticle pores in unsaturated and saturated porous media. While it has been demonstrated in experiments that the gas-water interface poses a high potential for retardation (Brusseau et al, 1997;Costanza-Robinson and Brusseau, 2002b), determining the effective interfacial area, which controls the contaminant mass transfer, is still subject of intensive research (Goss, 2009;Kibbey and Chen, 2012;Brusseau et al, 2015). Mayes et al (2003) stated that immobile water in pores could act as a short-term sink and as a long-term source of potential contaminants.…”