Natural Attenuation Conditions Research Articles

Microbial community evolution during the aerobic biodegradation of petroleum hydrocarbons in marine sediment microcosms: Effect of biostimulation and seasonal variations

Evolution of the microbial community structure in crude oil contaminated marine sediments was assessed under aerobic biodegradation during wet (18 °C) and dry (28 °C) seasons experiments, to account for seasonal variations in nutrients and temperature, under biostimulation and natural attenuation conditions. NMDS showed significant variation in the microbial communities between the wet and the dry season experiments, and between the biostimulation and the natural attenuation treatments in the dry season microcosms. No significant variation in the microbial community and oil biodegradation was observed during the wet season experiments due to high background nitrogen levels eliminating the effect of biostimulation. Larger variations were observed in the dry season experiments and were correlated to enhanced alkanes removal in the biostimulated microcosms, where Alphaproteobacteria dominated the total microbial community by the end of biodegradation (54%). Many hydrocarbonoclastic bacterial genera showed successive dominance during the operation affecting the ultimate performance of the microcosms.

Biodegradation of Chlorothalonil Fungicide in Coastal Areas of the Colombian Caribbean Suitable for Banana Crops

Chlorothalonil (CHT) is a chlorinated-nitrogen fungicide used in the control of black sigatoka, a disease that affects banana and plantain, has adverse effects on the environment and its metabolites can be toxic. The CHT has been the object of several studies related to its behavior and degradation in the environment; however, few works have been developed in tropical areas, such as the case of the banana zone of Magdalena - Colombia. In addition, it is unknown if the microbial diversity existing in these soils can transform compounds such as CHT, thus these microorganisms can be the basis for the implementation of remediation strategies based on bioaugmentation. In this sense, the present study aims to evaluate the degradation of CHT in soils of a tropical coastal zone, under conditions of natural attenuation and bioaugmentation with native microorganisms. For this purpose, microorganisms capable of growing with CHT as their sole carbon source (10.8 mg.L-1) and tolerating a concentration up to 25 mg.L-1 were isolated and selected from soils in the Magdalena-Colombia banana zone. Subsequently, CHT degradation tests (2000 ng.g-1) were performed on experimental plots, with bioaugmentation of native microorganisms and natural attenuation during 35 days, evaluating the growth of microbial populations and the degradation of CHT by gas chromatography associated with a mass spectrometry detector (GC/MS). Because of the microbial selection process, 8 morphotypes were obtained with the ability to grow in CHT as the sole carbon source and tolerate concentrations up to 25 g.L-1. The isolates were identified by sequencing the 16S rDNA gene as belonging to the genera Enterobacter, Klebsiella, Pseudomonas and Citrobacter. The CHT degradation tests showed that under bioaugmentation conditions with the native microorganisms selected in the study, a 100% reduction was achieved after 21 days; while under natural attenuation conditions after 35 days the degradation reached values of 98.5%. The application of bioaugmentation of selected and enhanced bacteria increased the rate of pesticide degradation by ~2.5 times, reducing the half-life from 6.2 to 2.5 days. The results demonstrated the effectiveness of the bioaugmentation treatment and that the selected consortium may be useful for bioremediation of banana crops soils contaminated with the fungicide.

Assessment of crude oil bioremediation potential of seawater and sediments from the shore of Lebanon in laboratory microcosms

With the planned oil and gas exploration activities off the coast of Lebanon, the risk of shoreline contamination with crude oil spills has become a major concern. This study aimed at assessing the crude oil bioremediation potential of the chronically polluted Lebanese shores in light of the continuous discharge of nutrient-rich sewage into the Mediterranean Sea and the long-lasting absence of proper sewage treatment systems. It was anticipated that, with the high pollution levels of the coastline, background concentrations of nutrients would be sufficient to sustain high intrinsic biodegradation rates without human intervention. Biodegradation experiments were conducted using crude oil-spiked beach sediments and seawater under natural attenuation and biostimulation conditions. The experiments were conducted at 18 and 28 °C to account for seasonal variation in temperature, background nutrient levels, and microbial communities. The biodegradability of oil constituents – namely alkanes and polycyclic aromatic hydrocarbons (PAHs), was monitored over a 42-day period using gas chromatography–mass spectrometry (GC–MS). Under biostimulation conditions, significant enhancement in the overall biodegradation rates of alkanes and PAHs was observed in seawater at 18 and 28 °C, while little to no improvement was measured at both temperatures in sediments where background nutrient levels were sufficient to induce near maximum intrinsic biodegradation rates. Under both natural attenuation and biostimulation treatments, the increase in temperature increased the oil biodegradation rates in sediment and seawater microcosms. In both instances, the overall trend in the biodegradation of individual alkanes and PAHs suggested a typical decrease in biodegradation rates with the increase in carbon number/rings and alkyl groups.

In-situ biodegradation potential of 1,2-DCA and VC at sites with different hydrogeological settings

This paper investigates the feasibility of applying in-situ Bioremediation (ISB) to three sites contaminated with vinyl chloride and/or chlorinated alkanes such as 1,2-DCA and 1,1,2-TCA, presenting distinct hydrogeological settings and history of contaminant loading. Biotransformation of these compounds is well established in laboratory studies and pure cultures. Due to confidential aspects, however, few field data are available to support real case studies to the predictability of their fate and lifetime in soil and groundwater. Bio-Trap® In Situ Microcosm (ISM) studies were performed in selected monitoring wells, and consisted of a control unit which simulated Monitored Natural Attenuation conditions and other units which were amended with either lactate, emulsified vegetable oil (EVO) or molasses as electron donors. For wells with moderate Dhc counts, the ISM study demonstrated that electron donor addition could stimulate further growth of Dhc and enhance reductive dechlorination. Conversely, for wells with high population counts, substrate addition did not alter results significantly. Site-specific determining factors that most influenced the biodegradation results were microbial activity, soil texture and presence of organic matter, site pH, redox conditions and presence of free phase.

Dynamic Processes in Capillary Fringes

CSA NewsVolume 60, Issue 6 p. 23-23 News & PerspectiveOpen Access Dynamic Processes in Capillary Fringes First published: 09 June 2015 https://doi.org/10.2134/csa2015-60-6-8AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL The May 2015 issue of Vadose Zone Journal features a special section on processes in capillary fringes, with a focus on the complex interaction of biological, chemical, and physical processes in this environemnt. Capillary fringes are highly dynamic and periodically changing zones at the interface between the water-saturated aquifer and the unsaturated zone. As a consequence, we observe steep gradients in terms of hydraulic state variables, biogeochemical conditions, and thus, biological activity (Berkowitz et al., 2004). With fluctuations of the groundwater table, this reactive zone moves dynamically along the vertical direction, further complicating our understanding of specific processes that occur in both space and time. This special compartment is not only an interesting and challenging subject of research, it is also highly relevant for mass fluxes with terrestrial systems (Silliman et al., 2002; Haberer et al., 2011) and especially the biodegradation of contaminants (Lahvis et al., 1999; Dobson et al., 2007). To gain a comprehensive understanding of the mass transfer and transformation in the CF, the close interactions of physical, chemical, and biological processes call for an interdisciplinary approach. This was the motivation to establish the interdisciplinary research group DyCap, funded by the Deutsche Forschungsgemeinschaft for a period of 6 years. The aim was to explore the basic physical, (hydro-)geological, chemical, and microbiological processes in CFs and their interaction, while collaborating closely with an interdisciplinary team of researchers. This special issue contains some of the latest results of DyCap, together with other actual research related to the dynamics of capillary fringes. This special issue presents examples of complex processes within capillary fringes. The contributions demonstrate the intensive coupling of biological, chemical, and physical processes relevant for a profound understanding of mass fluxes and turnover within this hot spot at the surface of the groundwater table. We hope these papers and increased knowledge will inspire future research on dynamic capillary fringes. References Berkowitz, B., Silliman, S.E., and Dunn, A.M. 2004. Impact of the capillary fringe on local flow, chemical migration, and microbiology. Vadose Zone J. 3: 534– 548. https://doi.org/10.2113/3.2.534 Dobson, R., Schroth, M.H., and Zeyer, J. 2007. Effect of water-table fluctuation on dissolution and biodegradation of a multi-component, light nonaqueous-phase liquid. J. Contam. Hydrol. 94: 235– 248. https://doi.org/10.1016/j.jconhyd.2007.07.007 Haberer, C.M., Rolle, M., Liu, S., Cirpka, O.A., and Grathwohl, P. 2011. A high-resolution non-invasive approach to quantify oxygen transport across the capillary fringe and within the underlying groundwater. J. Contam. Hydrol. 122: 26– 39. https://doi.org/10.1016/j.jconhyd.2010.10.006 Lahvis, M.A., Baehr, A.L., and Baker, R.J. 1999. Quantification of aerobic biodegradation and volatilization rates of gasoline hydrocarbons near the water table under natural attenuation conditions. Water Resour. Res. 35: 753– 765. https://doi.org/10.1029/1998WR900087 Silliman, S.E., Berkowitz, B., Šimůnek, J., and Genuchten, M.T. 2002. Fluid flow and solute migration within the capillary fringe. Ground Water 40: 76– 84. https://doi.org/10.1111/j.1745-6584.2002.tb02493.x Volume60, Issue6June 2015Pages 23-23 ReferencesRelatedInformation

Nitrate addition to groundwater impacted by ethanol-blended fuel accelerates ethanol removal and mitigates the associated metabolic flux dilution and inhibition of BTEX biodegradation

A comparison of two controlled ethanol-blended fuel releases under monitored natural attenuation (MNA) versus nitrate biostimulation (NB) illustrates the potential benefits of augmenting the electron acceptor pool with nitrate to accelerate ethanol removal and thus mitigate its inhibitory effects on BTEX biodegradation. Groundwater concentrations of ethanol and BTEX were measured 2m downgradient of the source zones. In both field experiments, initial source-zone BTEX concentrations represented less than 5% of the dissolved total organic carbon (TOC) associated with the release, and measurable BTEX degradation occurred only after the ethanol fraction in the multicomponent substrate mixture decreased sharply. However, ethanol removal was faster in the nitrate amended plot (1.4years) than under natural attenuation conditions (3.0years), which led to faster BTEX degradation. This reflects, in part, that an abundant substrate (ethanol) can dilute the metabolic flux of target pollutants (BTEX) whose biodegradation rate eventually increases with its relative abundance after ethanol is preferentially consumed. The fate and transport of ethanol and benzene were accurately simulated in both releases using RT3D with our general substrate interaction module (GSIM) that considers metabolic flux dilution. Since source zone benzene concentrations are relatively low compared to those of ethanol (or its degradation byproduct, acetate), our simulations imply that the initial focus of cleanup efforts (after free-product recovery) should be to stimulate the degradation of ethanol (e.g., by nitrate addition) to decrease its fraction in the mixture and speed up BTEX biodegradation.

Assessing impediments to hydrocarbon biodegradation in weathered contaminated soils

In this study, impediments to hydrocarbon biodegradation in contaminated soils were assessed using chemical and molecular methodologies. Two long-term hydrocarbon contaminated soils were utilised which were similar in physico-chemical properties but differed in the extent of hydrocarbon (C10–C40) contamination (S1: 16.5gkg−1; S2: 68.9gkg−1). Under enhanced natural attenuation (ENA) conditions, hydrocarbon biodegradation was observed in S1 microcosms (26.4% reduction in C10–C40 hydrocarbons), however, ENA was unable to stimulate degradation in S2. Although eubacterial communities (PCR-DGGE analysis) were similar for both soils, the alkB bacterial community was less diverse in S2 presumably due to impacts associated with elevated hydrocarbons. When hydrocarbon bioaccessibility was assessed using HP-β-CD extraction, large residual concentrations remained in the soil following the extraction procedure. However, when linear regression models were used to predict the endpoints of hydrocarbon degradation, there was no significant difference (P>0.05) between HP-β-CD predicted and microcosm measured biodegradation endpoints. This data suggested that the lack of hydrocarbon degradation in S2 resulted primarily from limited hydrocarbon bioavailability.

Bioremediation of diesel-polluted soil using biostimulation as post-treatment after oxidation with Fenton-like reagents: Assays in a pilot plant

The present study focuses on the remediation of diesel-polluted soil using modified Fenton treatment coupled with inorganic NPK fertilizer (“Fenton+NPK”). Studies were carried out in a pilot plant containing 1m3 of sandy soil contaminated with 20,000mgkg−1 of diesel, placed outdoors at a temperature ranging between 5 and 10°C. Results showed that NPK-fertilizer as post-treatment stimulated culturable degrading bacteria and enhanced dehydrogenase activity. Fenton+NPK treatment increased total petroleum hydrocarbon (TPH) removal efficacy. Natural attenuation removed 49% of TPH in the surface layer, 23% of TPH in the non-saturated layer and 4% of the TPH in the saturated layer, while the percentage removed of TPH after Fenton+NPK treatment was 58%, 57% and 32% respectively. The results from our study showed that, immediately after soil contamination, occurred a specialization and differentiation of the bacterial community, but after this initial modification, no significant changes of bacterial diversity was observed under natural attenuation conditions. In contrast, when the Fenton's reagent was applied a reduction of the bacterial biodiversity was observed. However, the post-biostimulation did enhance the degrading microbiota and stimulated their degrading biological activity. In conclusion, biostimulation, as a post-treatment step in chemical oxidation, is an effective solution to remediate hydrocarbon-polluted sites.

Detecting and quantifying reductive dechlorination under monitored natural attenuation conditions

AbstractField sampling and testing were used to investigate the relationship between baseline geochemical and microbial community data and in situ reductive dechlorination rates at a site contaminated with trichloroethene (TCE) and carbon tetrachloride (CTET). Ten monitoring wells were selected to represent conditions along groundwater flow paths from the contaminant source zone to a wetlands groundwater discharge zone. Groundwater samples were analyzed for a suite of geochemical and microbial parameters; then push‐pull tests with fluorinated reactive tracers were conducted in each well to measure in situ reductive dechlorination rates. No exogenous electron donors were added in these tests, as the goal was to assess in situ reductive dechlorination rates under natural attenuation conditions. Geochemical data provided preliminary evidence that reductive dechlorination of TCE and CTET was occurring at the site, and microbial data confirmed the presence of known dechlorinating organisms in groundwater. Push‐pull tests were conducted using trichlorofluoroethene (TCFE) as a reactive tracer for TCE and, in one well, trichlorofluoromethane (TCFM) as a reactive tracer for CTET. Injected TCFE was transformed to cis‐ and trans‐dichlorofluoroethene and chlorofluoroethene, and, in one test, injected TCFE was completely dechlorinated to fluoroethene (FE). In situ TCFE transformation rates ranged from less than 0.005 to 0.004/day. In the single well tested, injected TCFM was transformed in situ to dichlorofluoromethane and chlorofluoromethane; the TCFM transformation rate was estimated as 0.001/day. The results indicate that it is possible to use push‐pull tests with reactive tracers to directly detect and quantify reductive dechlorination of chlorinated ethenes and ethanes under monitored natural attenuation conditions, which has not previously been demonstrated. Transformation rate estimates obtained with these techniques should improve the accuracy of contaminant transport modeling. © 2012 Wiley Periodicals, Inc.

Impact of bacterial and fungal processes on 14C-hexadecane mineralisation in weathered hydrocarbon contaminated soil

In this study, the impact of bacterial and fungal processes on 14C-hexadecane mineralisation was investigated in weathered hydrocarbon contaminated soil. The extent of 14C-hexadecane mineralisation varied depending on the bioremediation strategy employed. Under enhanced natural attenuation conditions, 14C-hexadecane mineralisation after 98days was 8.5±3.7% compared to <1.2% without nitrogen and phosphorus additions. 14C-hexadecane mineralisation was further enhanced through Tween 80 amendments (28.9±2.4%) which also promoted the growth of a Phanerochaete chyrsosporium fungal mat. Although fungal growth in weathered hydrocarbon contaminated soil could be promoted through supplementing additional carbon sources (Tween 80, sawdust, compost, pea straw), fungal 14C-hexadecane mineralisation was negligible when sodium azide was added to soil microcosms to inhibit bacterial activity. In contrast, when fungal activity was inhibited through nystatin additions, 14C-hexadecane mineralisation ranged from 6.5±0.2 to 35.8±3.8% after 98days depending on the supplied amendment. Bacteria inhibition with sodium azide resulted in a reduction in bacterial diversity (33–37%) compared to microcosms supplemented with nystatin or microcosms without inhibitory supplements. However, alkB bacterial groups were undetected in sodium azide supplemented microcosms, highlighting the important role of this bacterial group in 14C-hexadecane mineralisation.

1,4-Dioxane biodegradation at low temperatures in Arctic groundwater samples

1,4-Dioxane biodegradation was investigated in microcosms prepared with groundwater and soil from an impacted site in Alaska. In addition to natural attenuation conditions (i.e., no amendments), the following treatments were tested: (a) biostimulation by addition of 1-butanol (a readily available auxiliary substrate) and inorganic nutrients; and (b) bioaugmentation with Pseudonocardia dioxanivorans CB1190, a well-characterized dioxane-degrading bacterium, or with Pseudonocardia antarctica DVS 5a1, a bacterium isolated from Antarctica. Biostimulation enhanced the degradation of 50 mg L −1 dioxane by indigenous microorganisms (about 0.01 mg dioxane d −1 mg protein −1) at both 4 and 14 °C, with a simultaneous increase in biomass. A more pronounced enhancement was observed through bioaugmentation. Microcosms with 50 mg L −1 initial dioxane (representing source-zone contamination) and augmented with CB1190 degraded dioxane fastest (0.16 ± 0.04 mg dioxane d −1 mg protein −1) at 14 °C, and the degradation rate decreased dramatically at 4 °C (0.021 ± 0.007 mg dioxane d −1 mg protein −1). In contrast, microcosms with DVS 5a1 degraded dioxane at similar rates at 4 °C and 14 °C (0.018 ± 0.004 and 0.015 ± 0.006 mg dioxane d −1 mg protein −1, respectively). DVS 5a1 outperformed CB1190 when the initial dioxane concentration was low (500 μg L −1, which is representative of the leading edge of plumes). This indicates differences in competitive advantages of these two strains. Natural attenuation microcosms also showed significant degradation over 6 months when the initial dioxane concentration was 500 μg L −1. This is the first study to report the potential for dioxane bioremediation and natural attenuation of contaminated groundwater in sensitive cold-weather ecosystems such as the Arctic.

Chromium speciation and existing natural attenuation conditions in lagoonal and pond sediments in the former chemical plant of Porto-Romano (Albania)

Near the outskirts of the Albanian port of Durres on the Adriatic Coast, at a place called Porto Romano, an old chemical complex has been producing chromate salts for a period of ∼20 years (1972–1993). As a result large quantities of chromite ore processing residue (COPR) waste were released mainly as suspended particles and/or dissolved species in effluents, which settled in a decantation pond area, 200 × 250 m in size, east of the industrial plant. Unfortunately, pollutants were spread out with time. Knowing that chromium mobility and toxicity in natural settings is species dependent, the species characterization has been carried out in different sampling media (pond sediments, lagoonal sediments and water samples). Five pond profiles were sampled within the decantation area up to a depth of ∼1 m, till the underlying Quaternary lagoonal silty-clays were reached. Total Cr content is highly variable with values between 1,130 and 24,409 mg/kg which is clearly higher than the defined local background values for lagoonal silty-clays (198 mg/kg of Cr). Leaching of Cr(VI) especially occurs in the pond sediments, which are low in OC, CEC and clay content, under acidic and neutral pH conditions (e.g. up to ∼2,230 mg/kg Cr(VI) leached from a sample containing a total concentration of Cr 12,200 mg/kg). Moreover, leaching of the Cr(III) occurs only under strictly acidic conditions (maximum 1,144 mg/kg leached from a sample containing a total Cr-concentration of 17,608 mg/kg). In this study also a number of natural attenuation conditions (i.e. reaction with lagoonal clays rich in organic matter and iron as well as isomorphous substitution) have been recognized.

Factors controlling BTEX and chlorinated solvents plume length under natural attenuation conditions

Natural attenuation is presently used at numerous sites where groundwater is contaminated. In order to simulate this attenuation, reactive transport models are often used but they are quite complex and depend on both physical and chemical conditions in the aquifer. As complex numerical models cannot be used to study all possible cases, we develop here analytical solutions to draw general conclusions. Our strategy, called MIKSS (Mixed Instantaneous and Kinetics Superposition Sequence), allows the calculation of the concentrations of all reacting substances in a plume. It is an extension of the superimposition principle that is able to treat the case of joint kinetics and instantaneous reactions. The basic equations have been extended to treat different reactions that occur in the plume core and at its fringe. At first we consider one organic substance degraded under all oxidising conditions (toluene for instance). For this problem the size of the plume depends on the reduced source width and on the ratio of the organic substance concentration to the sum of the electron acceptors' concentrations. For several BTEX substances having different degradation behaviour the formulation is similar, but leads to quite different plume lengths for each substance. Contrary to the case of one substance, the plumes can be quite long and may not satisfy the target risk level. For chlorinated solvents we developed a specific approach to take under consideration all reactions and particularly the competition for hydrogen. A formula is given to assess the size of the plume core, i.e. the zone with highly reducing conditions. The factors influencing the core length are the same as for BTEX (source width, dispersivity, organic carbon content). The size of the TCE plume is calculated from the plume core length and the kinetic constant of TCE degradation. Using assumptions of degradation constants for DCE and VC it is also possible to calculate the longitudinal concentration profile of these substances. The degradation of moderately substituted solvents under oxic conditions reduces the size of their plumes but under these conditions TCE becomes the major threat. Among the conditions studied in this paper, very few chlorinated solvents sites can lead to a negligible risk at an acceptable distance from the source.

A Modeling Study and Implications of Competition between Dehalococcoides ethenogenes and Other Tetrachloroethene- Respiring Bacteria

Bioaugmentation with cultures containing Dehalococcoides strains that dechlorinate cis-1,2-dichloroethene (cDCE) to ethene is often recommended at sites where indigenous populations dechlorinate tetrachloroethene (PCE) to cDCE. In these cases, Dehalococcoides populations may have to compete with other dehalorespirers for chlorinated ethenes and/or electron donors. A continuous-flow stirred tank reactor model was used to describe the substrate interactions in three conceptual models of competition between PCE-respiring populations under natural attenuation and engineered bioremediation scenarios. Model simulations were used to evaluate the effects of other chlorinated ethene respirers on substrate utilization by, and growth of, a Dehalococcoides strain (Dhc. ethenogenes strain 195) and identify the key factors influencing the outcome of competition among different dehalorespirers. The results suggest that, under natural attenuation conditions, Dhc. ethenogenes is unlikely to be the dominant population if a dehalorespirer that can compete for limiting amounts of reducing equivalents is present. Engineered bioremediation conditions resulted in greater enrichment of Dhc. ethenogenes than of competing dehalorespirers. Under several conditions, Dhc. ethenogenes coexisted with a PCE-to-cDCE (or PCE-to-TCE) dehalorespirer, primarily by functioning as a cDCE- (or TCE)-to-ethene dechlorinating specialist. From a bioremediation standpoint, maintenance of multiple dehalorespiring specialists appears ideal because it may result in the fastest and most extensive chlorinated ethene transformations. Thus, to improve our ability to successfully implement bioremediation, it may be helpful to characterize the indigenous PCE-to-cDCE respiring populations and the nature and distribution of electron donors used by these dehalorespirers at contaminated sites. Further understanding of these interactions requires more accurate information on the kinetics of known dehalorespiring populations.

Methodology for Site-Specific, Mobility-Based Cleanup Standards for Heavy Metals in Glaciated Soils

A framework was developed for establishing site-specific, mobility-based cleanup standards for heavy metal contaminated sites that incorporates a detailed soil characterization, laboratory mobility studies, and mathematical modeling. This framework was applied to a chromium contaminated soil from a metal finishing facility. A series of column leaching experiments were conducted at two different flow rates and influent pHs. A mathematical model was used to describe the fate, speciation, and transport of heavy metals in porous media. In this model, dissolved heavy metals interact with oxide coatings and soil organic matter on the outer surfaces of soil grains via surface complexation reactions. In addition, dissolved heavy metals diffuse into the soil matrix and react with intragranular reactive sites. Model simulations were performed for chromium to determine apparent mass transfer coefficients. A long-term simulation of the unsaturated zone at the site was performed to estimate cleanup levels for chromium under natural attenuation conditions. Applying this framework for the site-specific, mobility-based determination of cleanup levels provides a solution to the problem of estimating a practical cleanup goal.

Quantification of aerobic biodegradation and volatilization rates of gasoline hydrocarbons near the water table under natural attenuation conditions

Aerobic biodegradation and volatilization near the water table constitute a coupled pathway that contributes significantly to the natural attenuation of hydrocarbons at gasoline spill sites. Rates of hydrocarbon biodegradation and volatilization were quantified by analyzing vapor transport in the unsaturated zone at a gasoline spill site in Beaufort, South Carolina. Aerobic biodgradation rates decreased with distance above the water table, ranging from 0.20 to 1.5 g m−3 d−1 for toluene, from 0.24 to 0.38 g m−3 d−1 for xylene, from 0.09 to 0.24 g m−3 d−1 for cyclohexene, from 0.05 to 0.22 g m−3 d−1 for ethylbenzene, and from 0.02 to 0.08 g m−3 d−1 for benzene. Rates were highest in the capillary zone, where 68% of the total hydrocarbon mass that volatilized from the water table was estimated to have been biodegraded. Hydrocarbons were nearly completely degraded within 1m above the water table. This large loss underscores the importance of aerobic biodradation in limiting the transport of hydrocarbon vapors in the unsaturated zone and implies that vapor‐plume migration to basements and other points of contact may only be significant if a source of free product is present. Furthermore, because transport of the hydrocarbon in the unsaturated zone can be limited relative to that of oxygen and carbon dioxide, soil‐gas surveys conducted at hydrocarbon‐spill sites would benefit by the inclusion of oxygen‐ and carbon‐dioxide‐gas concentration measurements. Aerobic degradation kinetics in the unsaturated zone were approximately first‐order. First‐order rate constants near the water table were highest for cyclohexene (0.21–0.65 d−1) and nearly equivalent for ethylbenzene (0.11–0.31 d−1), xylenes (0.10–0.31 d−1), toluene (0.09–0.30 d−1), and benzene (0.07–0.31 d−1). Hydrocarbon mass loss rates at the water table resulting from the coupled aerobic biodgradation and volatilization process were determined by extrapolating gas transport rates through the capillary zone. Mass loss rates from groundwater were highest for toluene (0.20–0.84 g m−2 d−1), followed by xylenes (0.12–0.69 g m−2 d−1), cyclohexene (0.05–0.15 g m−2 d−1), ethylbenzene (0.02–0.12 g m−2 d−1), and benzene (0.01–0.04 g m−2 d−1). These rates exceed predicted rates of solubilization to groundwater, demonstrating the effectiveness of aerobic biodgradation and volatilization as a combined natural attenuation pathway.