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1.
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.  相似文献   

2.
Detailed site investigations to assess potential inhalation exposure and risk to human health associated with the migration of petroleum hydrocarbon vapors from the subsurface to indoor air are frequently undertaken at leaking underground storage tank (UST) sites, yet documented occurrences of petroleum vapor intrusion are extremely rare. Additional assessments are largely driven by low screening‐level concentrations derived from vapor transport modeling that does not consider biodegradation. To address this issue, screening criteria were developed from soil‐gas measurements at hundreds of petroleum UST sites spanning a range of environmental conditions, geographic regions, and a 16‐year time period (1995 to 2011). The data were evaluated to define vertical separation (screening) distances from the source, beyond which, the potential for vapor intrusion can be considered negligible. The screening distances were derived explicitly from benzene data using specified soil‐gas screening levels of 30, 50, and 100 µg/m3 and nonparametric Kaplan‐Meier statistics. Results indicate that more than 95% of benzene concentrations in soil gas are ≤30 µg/m3 at any distance above a dissolved‐phase hydrocarbon source. Dissolved‐phase petroleum hydrocarbon sources are therefore unlikely to pose a risk for vapor intrusion unless groundwater (including capillary fringe) comes in contact with a building foundation. For light nonaqueous‐phase liquid (LNAPL) hydrocarbon sources, more than 95% of benzene concentrations in soil gas are ≤30 µg/m3 for vertical screening distances of 13 ft (4 m) or greater. The screening distances derived from this analysis are markedly different from 30 to 100 ft (10 to 30 m) vertical distances commonly found cited in regulatory guidance, even with specific allowances to account for uncertainty in the hydrocarbon source depth or location. Consideration of these screening distances in vapor intrusion guidance would help eliminate unnecessary site characterization at petroleum UST sites and allow more effective and sustainable use of limited resources.  相似文献   

3.
In this study, we present a petroleum vapor intrusion (PVI) tool implemented in Microsoft® Excel® using Visual Basic for Applications and integrated within a graphical interface. The latter helps users easily visualize two‐dimensional soil gas concentration profiles and indoor concentrations as a function of site‐specific conditions such as source strength and depth, biodegradation reaction rate constant, soil characteristics and building features. This tool is based on a two‐dimensional explicit analytical model that combines steady‐state diffusion‐dominated vapor transport in a homogeneous soil with a piecewise first‐order aerobic biodegradation model, in which rate is limited by oxygen availability. As recommended in the recently released United States Environmental Protection Agency's final PVI guidance, a sensitivity analysis and a simplified Monte Carlo uncertainty analysis are also included in the spreadsheet.  相似文献   

4.
Groundwater monitoring wells are present at most hydrocarbon release sites that are being assessed for cleanup. If screened across the vadose zone, these wells provide an opportunity to collect vapor samples that can be used in the evaluation of vapor movement and biodegradation processes occurring at such sites. This paper presents a low purge volume method (modified after that developed by the U.S. EPA) for sampling vapor from monitoring wells that is easy to implement and can provide an assessment of the soil gas total petroleum hydrocarbon (TPH) and O2 concentrations at the base of the vadose zone. As a result, the small purge method allows for sampling of vapor from monitoring wells to support petroleum vapor intrusion (PVI) risk assessment. The small purge volume method was field tested at the Hal's service station site in Green River, Utah. This site is well‐known for numerous soil gas measurements containing high O2 and high TPH vapor concentrations in the same samples which is inconsistent with well‐accepted biodegradation models for the vapor pathway. Using the low purge volume method, monitoring wells were sampled over, upgradient, and downgradient of the light nonaqueous phase liquid (LNAPL) footprint. Results from our testing at Hal's show that vapor from monitoring wells over LNAPL contained very low O2 and high TPH concentrations. In contrast, vapor from monitoring wells not over LNAPL contained high O2 and low TPH concentrations. The results of this study show that a low purge volume method is consistent with biodegradation models especially for sampling at sites where low permeability soils exist in and around a LNAPL source zone.  相似文献   

5.
Aerobic biodegradation can contribute significantly to the attenuation of petroleum hydrocarbons vapors in the unsaturated zone; however, most regulatory guidance for assessing potential human health risks via vapor intrusion to indoor air either neglect biodegradation in developing generic screening levels or allow for only one order of magnitude additional attenuation for aerobically degradable compounds, which may be overly conservative in some cases. This paper describes results from three-dimensional numerical model simulations of vapor intrusion for petroleum hydrocarbons to assess the influence of aerobic biodegradation on the attenuation factor for a variety of source concentrations and depths for residential buildings with basements and slab-on-grade construction. The simulations conducted in this study provide a framework for understanding the degree to which bioattenuation will occur under a variety of scenarios and provide insight into site conditions that will result in significant biodegradation. This improved understanding may be used to improve the conceptual model of contaminant transport, guide field data collection and interpretation, and estimate semi-site-specific attenuation factors for combinations of source concentrations, source depth, oxygen distribution, and building characteristics where site conditions reasonably match the scenarios simulated herein.  相似文献   

6.
Data requirements for assessing the significance of the soil vapor intrusion pathway are evolving, and the collection and interpretation of subslab and near-slab soil-gas samples are under discussion. The potential for different assessment paradigms for aerobically biodegradable and recalcitrant chemicals is also frequently debated. In this work, the soil-gas distribution beneath and around a slab-on-grade building overlying shallow (0.5 to >1.5 m below ground surface) petroleum hydrocarbon–impacted coarse alluvial soils was studied. The study spanned about 12 months, including the sampling of soil-gas hydrocarbon and oxygen concentrations, subslab soil vs. building pressure differentials and included weather conditions. Three-dimensional soil-gas concentration "snapshots" using samples from 79 soil-gas sampling points are presented here. Significant spatial variability was observed with hydrocarbon and oxygen concentrations ranging from about <0.01 to 200 mg/L and 0 to 21% v/v, respectively. The presence of oxygen and the depth to petroleum-impacted soils appeared to be the dominant factors in controlling the soil-gas distribution; the depletion of hydrocarbons over short lateral and vertical distances (<2 m) was observed in the well-oxygenated regions. Composition data suggest preferential biodegradation of lighter compounds at some points, as reflected in the ratio of the masses of chemicals eluting on the gas chromatography between methane and pentane (C1 and C5) and all others after pentane (>C5).  相似文献   

7.
The potential for in situ biodegradation of tert‐butyl alcohol (TBA) by creation of aerobic conditions in the subsurface with recirculating well pairs was investigated in two field studies conducted at Vandenberg Air Force Base. In the first experiment, a single recirculating well pair with bromide tracer and oxygen amendment successfully delivered oxygen to the subsurface for 42 d. TBA concentrations were reduced from approximately 500 μg/L to below the detection limit within the treatment zone and the treated water was detected in a monitoring transect several meters downgradient. In the second experiment, a site‐calibrated model was used to design a double recirculating well pair with oxygen amendment, which successfully delivered oxygen to the subsurface for 291 d and also decreased TBA concentrations to below the detection limit. Methylibium petroleiphilum strain PM1, a known TBA‐degrading bacterium, was detectable at the study site but addition of oxygen had little impact on the already low baseline population densities, suggesting that there was not enough carbon within the groundwater plume to support significant new growth in the PM1 population. Given favorable hydrogeologic and geochemical conditions, the use of recirculating well pairs to introduce dissolved oxygen into the subsurface is a viable method to stimulate in situ biodegradation of TBA or other aerobically degradable aquifer contaminants.  相似文献   

8.
Aerobic biodegradation of vapor-phase petroleum hydrocarbons was evaluated in an intact soil core from the site of an aviation gasoline release. An unsaturated zone soil core was subjected to a flow of nitrogen gas, oxygen, water vapor, and vapor-phase hydrocarbons in a configuration analogous to a biofilter or an in situ bioventing or sparging situation. The vertical profiles of vapor-phase hydrocarbon concentration in the soil core were determined by gas chromatography of vapor samples. Biodegradation reduced low influent hydrocarbon concentrations by 45 to 92 percent over a 0.6-m interval of an intact soil core. The estimated total hydrocarbon concentration was reduced by 75 percent from 26 to 7 parts per million. Steady-state concentrations were input to a simple analytical model balancing advection and first-order biodegradation of hydrocarbons. First-order rate constants for the major hydrocarbon compounds were used to calibrate the model to the concentration profiles. Rate constants for the seven individual hydrocarbon compounds varied by a factor of 4. Compounds with lower molecular weights, fewer methyl groups, and no quaternary carbons tended to have higher rate constants. The first-order rate constants were consistent with kinetic parameters determined from both microcosm and tubing cluster studies at the field site.  相似文献   

9.
This study evaluates the theory, and some practical aspects of using temperature measurements to assess aerobic biodegradation in hydrocarbon contaminated soil. The method provides an easily applicable alternative for quantifying the rate of biodegradation and/or evaluating the performance of in situ remediation systems. The method involves two nonintrusive procedures for measuring vertical temperature profiles down existing monitoring wells; one using a thermistor on a cable for one‐time measurements and the other using compact temperature data loggers deployed for 3‐month to 1‐year period. These vertical temperature profile measurements are used to identify the depth and lateral extent of biodegradation as well as to monitor seasonal temperature changes throughout the year. The basic theory for using temperature measurements to estimate the minimum rate of biodegradation will be developed, and used to evaluate field measurements from sites in California where biodegradation of spilled petroleum hydrocarbons is due to natural processes. Following, temperature data will be used to evaluate the relative rates of biodegradation due to natural processes and soil vapor extraction (SVE) at a former refinery site in the North‐Central United States. The results from this study show that the temperature method can be a simple, cost effective tool for assessing biodegradation in the soil, and optimizing remediation systems at a wide variety of hydrocarbon spill sites.  相似文献   

10.
The fate of hydrocarbons in the subsurface near Bemidji, Minnesota, has been investigated by a multidisciplinary group of scientists for over a quarter century. Research at Bemidji has involved extensive investigations of multiphase flow and transport, volatilization, dissolution, geochemical interactions, microbial populations, and biodegradation with the goal of providing an improved understanding of the natural processes limiting the extent of hydrocarbon contamination. A considerable volume of oil remains in the subsurface today despite 30 years of natural attenuation and 5 years of pump‐and‐skim remediation. Studies at Bemidji were among the first to document the importance of anaerobic biodegradation processes for hydrocarbon removal and remediation by natural attenuation. Spatial variability of hydraulic properties was observed to influence subsurface oil and water flow, vapor diffusion, and the progression of biodegradation. Pore‐scale capillary pressure‐saturation hysteresis and the presence of fine‐grained sediments impeded oil flow, causing entrapment and relatively large residual oil saturations. Hydrocarbon attenuation and plume extent was a function of groundwater flow, compound‐specific volatilization, dissolution and biodegradation rates, and availability of electron acceptors. Simulation of hydrocarbon fate and transport affirmed concepts developed from field observations, and provided estimates of field‐scale reaction rates and hydrocarbon mass balance. Long‐term field studies at Bemidji have illustrated that the fate of hydrocarbons evolves with time, and a snap‐shot study of a hydrocarbon plume may not provide information that is of relevance to the long‐term behavior of the plume during natural attenuation.  相似文献   

11.
Sulfate reducing conditions are widely observed in groundwater plumes associated with petroleum hydrocarbon releases. This leads to sulfate depletion in groundwater which can limit biodegradation of hydrocarbons (usually benzene, toluene, ethylbenzene, xylenes [BTEX] compounds) and can therefore result in extended timeframes to achieve groundwater cleanup objectives by monitored natural attenuation. Under these conditions, sulfate addition to the subsurface can potentially enhance BTEX biodegradation and facilitate enhanced natural attenuation. However, a delivery approach that enables effective contact with the hydrocarbons and is able to sustain elevated and uniform sulfate concentrations in groundwater remains a key challenge. In this case study, sulfate addition to a groundwater plume containing predominantly benzene by land application of agricultural gypsum and Epsom salt is described. Over 4 years of groundwater monitoring data from key wells subjected to pilot‐scale and site‐wide land application events are presented. These are compared to data from pilot testing employing liquid Epsom salt injections as an alternate sulfate delivery approach. Sulfate land application, sulfate retention within the vadose zone, and periodic infiltration following ongoing precipitation events resulted in elevated sulfate concentrations (>150 mg/L) in groundwater that were sustained over 12 months between application events and stimulated benzene biodegradation as indicated by declines in dissolved benzene concentration, and compound‐specific isotope analysis data for carbon in benzene. Long‐term groundwater benzene concentration reductions were achieved in spite of periodic rebounds resulting from water table fluctuations across the smear zone. Land application of gypsum is a potentially cost‐effective sulfate delivery approach at sites with open, unpaved surfaces, relatively permeable geology, and shallow hydrocarbon impacts. However, more research is needed to understand the fate and persistence of sulfate and to improve the likelihood of success and effectiveness of this delivery approach.  相似文献   

12.
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13.
Vapor intrusion (VI) involves migration of volatile contaminants from subsurface through unsaturated soil into overlying buildings. In 2015, the US EPA recommended an approach for screening VI risks associated with gasoline releases from underground storage tank (UST) sites. Additional assessment of the VI risk from petroleum hydrocarbons was deemed unnecessary for buildings separated from vapor sources by more than recommended vertical screening distances. However, these vertical screening distances did not apply to potential VI risks associated with releases of former leaded gasoline containing 1,2-dichloroethane (1,2-DCA), because of a lack of empirical data on the attenuation of 1,2-DCA in soil gas. This study empirically evaluated 144 paired measurements of 1,2-DCA concentrations in soil gas and groundwater collected at 47 petroleum UST sites combined with BioVapor modeling. This included (1) assessing the frequency of 1,2-DCA detections in soil gas below 10−6 risk-based screening levels at different vertical separation distances and (2) comparing the US EPA recommended vertical screening distances with those predicted by BioVapor modeling. Vertical screening distances were predicted for different soil types using aerobic biodegradation rate constants estimated from the measured soil-gas data combined with conservative estimates of source concentrations. The modeling indicates that the vertical screening distance of 6 feet (1.8 m) recommended for dissolved-phase sources is applicable for 1,2-DCA below certain threshold concentrations in groundwater, while 15 feet (4.6 m) recommended for light nonaqueous phase liquid (LNAPL) sources is applicable for sites with clay and loam soils in the vadose zone, but not sand, if 1,2-DCA concentrations in groundwater exceed 150 μg/L. This dependence of the predicted vertical screening distances on soil type places added emphasis on proper soil characterization for VI screening at sites with 1,2-DCA sources. The soil-gas data suggests that a vertical screening distance of 15 feet (4.6 m) is necessary for both dissolved-phase and LNAPL sources.  相似文献   

14.
Monitored natural attenuation is widely applied as a remediation strategy at hydrocarbon spill sites. Natural attenuation relies on biodegradation of hydrocarbons coupled with reduction of electron acceptors, including solid phase ferric iron (Fe(III)). Because arsenic (As) adsorbs to Fe‐hydroxides, a potential secondary effect of natural attenuation of hydrocarbons coupled with Fe(III) reduction is a release of naturally occurring As to groundwater. At a crude‐oil‐contaminated aquifer near Bemidji, Minnesota, anaerobic biodegradation of hydrocarbons coupled to Fe(III) reduction has been well documented. We collected groundwater samples at the site annually from 2009 to 2013 to examine if As is released to groundwater and, if so, to document relationships between As and Fe inside and outside of the dissolved hydrocarbon plume. Arsenic concentrations in groundwater in the plume reached 230 µg/L, whereas groundwater outside the plume contained less than 5 µg/L As. Combined with previous data from the Bemidji site, our results suggest that (1) naturally occurring As is associated with Fe‐hydroxides present in the glacially derived aquifer sediments; (2) introduction of hydrocarbons results in reduction of Fe‐hydroxides, releasing As and Fe to groundwater; (3) at the leading edge of the plume, As and Fe are removed from groundwater and retained on sediments; and (4) downgradient from the plume, patterns of As and Fe in groundwater are similar to background. We develop a conceptual model of secondary As release due to natural attenuation of hydrocarbons that can be applied to other sites where an influx of biodegradable organic carbon promotes Fe(III) reduction.  相似文献   

15.
The occurrence of aerobic biodegradation in the vadose zone between a subsurface source and a building foundation can all-but eliminate the risks from methane and petroleum vapor intrusion (PVI). Understanding oxygen availability and the factors that affect it (e.g., building sizes and their distribution) are therefore critical. Uncovered ground surfaces allow oxygen access to the subsurface to actively biodegrade hydrocarbons (inclusive of methane). Buildings can reduce the net flux of oxygen into the subsurface and so reduce degradation rates. Here we determine when PVI and methane risk is negligible and/or extinguished; defined by when oxygen is present across the entire sub-slab region of existing or planned slab-on-ground buildings. We consider all building slab sizes, all depths to vapor sources and the effect of spacings between buildings on the availability of oxygen in the subsurface. The latter becomes critical where buildings are in close proximity or when increased building density is planned. Conservative assumptions enable simple, rapid and confident screening should sites and building designs comply to model assumptions. We do not model the aboveground “building” processes (e.g., air exchange), and assume the slab-on-ground seals the ground surface so that biodegradation of hydrocarbons is minimized under the built structure (i.e., the assessment remains conservative). Two graphs represent the entirety of the outcomes that allow simple screening of hydrocarbon vapors based only on the depth to the source of vapors below ground, the concentration of vapors within the source, the width of the slab-on-ground building, and the gap between buildings; all independent of soil type. Rectangular, square, and circular buildings are considered. Comparison with field sites and example applications are provided, along with a simple 8-step screening guide set in the context of existing guidance on PVI assessment.  相似文献   

16.
Vast regions of the northern hemisphere are exposed to snowfall and seasonal frost. This has large effects on spatiotemporal distribution of infiltration and groundwater recharge processes as well as on the fate of pollutants. Therefore, snow and frost need to be central inherent elements of risk assessment and management schemes. However, snow and frost are often neglected or treated summarily or in a simplistic way by groundwater modellers. Snow deposition is uneven, and the snow is likely to sublimate, be redistributed and partly melt during the winter influencing the mass and spatial distribution of snow storage available for infiltration, the presence of ice layers within and under the snowpack and, therefore, also the spatial distribution of depths and permeability of the soil frost. In steep terrain, snowmelt may travel downhill tens of metres in hours along snow layers. The permeability of frozen soil is mainly influenced by soil type, its water and organic matter content, and the timing of the first snow in relation to the timing of sub‐zero temperatures. The aim with this paper is to review the literature on snow and frost processes, modelling approaches with the purpose to visualize and emphasize the need to include these processes when modelling, managing and predicting groundwater recharge for areas exposed to seasonal snow and frost. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

17.
The tetrachloroethene (PCE) source zone at a site in Endicott, New York had caused a dissolved PCE plume. This plume was commingled with a petroleum hydrocarbon plume from an upgradient source of fuel oil. The plume required a system for hydraulic containment, using extraction wells located about 360 m downgradient of the source. The source area was remediated using in situ thermal desorption (ISTD). Approximately 1406 kilograms (kg) of PCE was removed in addition to 4082 kg of commingled petroleum‐related compounds. The ISTD treatment reduced the PCE mass discharge into the plume from an estimated 57 kg/year to 0.07 kg/year, essentially removing the source term. In the 5 years following the completion of the thermal treatment in early 2010, the PCE plume has collapsed, and the concentration of degradation products in the PCE‐series plume area has declined by two to three orders of magnitude. Anaerobic dechlorination is the suspected dominant mechanism, assisted by the presence of a fuel oil smear zone and a petroleum hydrocarbon plume from a separate source area upgradient of the PCE source. Based on the post‐thermal treatment groundwater monitoring data, the hydraulic containment system was reduced in 2014 and discontinued in early 2015.  相似文献   

18.
Vapor intrusion (VI) occurs when volatile contaminants in the subsurface migrate through the vadose zone into overlying buildings. The 2015 U.S. EPA petroleum VI guidance recommends that additional investigation of the VI risk from gasoline hydrocarbons at the underground storage tank (UST) sites is not necessary where the vertical distance between a building and a vapor source exceeds a recommended vertical screening distance. However, due to the lack of soil-gas data on the attenuation of ethylene dibromide (EDB), additional VI investigations to evaluate VI risk from EDB are recommended at UST sites with leaded gasoline releases containing EDB. We analyzed soil-gas and groundwater concentrations of EDB from eight petroleum UST sites using a new analytical method with soil-gas detection limit <0.16 μg/m3 EDB (VI screening level at the 10−6 risk level). The analysis included (1) assessing the frequency of EDB detections ≤0.16 μg/m3 at various vertical separation distances and (2) predicting vertical screening distances for EDB using the U.S. EPA PVIScreen model for different soil types in the vadose zone above dissolved-phase and LNAPL sources. Ranges of estimated aerobic biodegradation rate constants for EDB, air exchange rates for residential buildings, and source vapor concentrations for other constituents were combined with conservative estimates of EDB source concentrations as model inputs. Concentrations of EDB in soil-gas indicated that the U.S. EPA recommended vertical screening distances are protective of VI risk from EDB. Conversely, vertical screening distances predicted by modeling were >6 ft (1.8 m) for sites with sand and loam soil above dissolved phase sources and >15 ft (4.6 m) for sites with sand soil above LNAPL sources. This predicted dependence on the vapor source type and soil type in the vadose zone highlights the importance of soil characterization for VI screening at sites with EDB sources.  相似文献   

19.
Vapor intrusion pathway evaluations commonly begin with a comparison of volatile organic chemical (VOC) concentrations in groundwater to generic, or Tier 1, screening levels. These screening levels are typically quite low reflecting both a desired level of conservatism in a generic risk screening process as well as limitations in understanding of physical and chemical processes that impact vapor migration in the subsurface. To study the latter issue, we have collected detailed soil gas and groundwater vertical concentration profiles and evaluated soil characteristics at seven different sites overlying chlorinated solvent contaminant plumes. The goal of the study was to evaluate soil characteristics and their impacts on VOC attenuation from groundwater to deep soil gas (i.e., soil gas in the unsaturated zone within 2 feet of the water table). The study results suggest that generic screening levels can be adjusted by a factor of 100× at sites with fine‐grained soils above the water table, as identified by visual observations or soil air permeability measurements. For these fine‐grained soil sites, the upward‐adjusted screening levels maintain a level of conservatism while potentially eliminating the need for vapor intrusion investigations at sites that may not meet generic screening criteria.  相似文献   

20.
Geochemical Indicators of Intrinsic Bioremediation   总被引：19，自引：0，他引：19  
A detailed field investigation has been completed at a gasoline-contaminated aquifer near Rocky Point, NC, to examine possible indicators of intrinsic bioremediation and identify factors that may significantly influence the rate and extent of bioremediation. The dissolved plume of benzene, toluene, ethylbenzene, and xylene (BTEX) in ground water is naturally degrading. Toluene and o-xylene are most rapidly degraded followed by m-, p-xylene, and benzene. Ethylbenzene appears to degrade very slowly under anaerobic conditions present in the center of the plume. The rate and extent of biodegradation appears to be strongly influenced by the type and quantity of electron acceptors present in the aquifer. At the upgradient edge of the plume, nitrate, ferric iron, and oxygen are used as terminal electron acceptors during hydrocarbon biodegradation. The equivalent of 40 to 50 mg/I of hydrocarbon is degraded based on the increase in dissolved CO2 relative to background ground water. Immediately downgradient of the source area, sulfate and iron are the dominant electron acceptors. Toluene and o-xylene are rapidly removed in this region. Once the available oxygen, nitrate, and sulfate are consumed, biodegradation is limited and appears to be controlled by mixing and aerobic biodegradation at the plume fringes.  相似文献   

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