Methyl Tert-butyl Ether Plume Research Articles

Monitoring In Situ Biodegradation of MTBE Using Multiple Rounds of Compound‐Specific Stable Carbon Isotope Analysis

AbstractAt a large industrial facility, methyl tert‐butyl ether (MTBE) was released to the subsurface and dispersed into the light, non‐aqueous phase liquids (LNAPL), in the first aquifer, with the LNAPL serving as a continuous source of MTBE in groundwater. Compound‐specific isotope analysis was conducted on both MTBE and tert‐butyl alcohol (TBA) in groundwater samples collected in 2008, 2011, and 2013 from wells located along and off the center line of the MTBE plume. The study demonstrated the onset and progress of biodegradation of MTBE between 2008 and 2013. The TBA observed in 2008 appears to be derived only in part from MTBE transformation while a significant portion of TBA might be contributed directly from LNAPL sources. In 2011 to 2013, the dominant source of TBA in the mid‐gradient plume was MTBE transformation. A contribution of an offsite LNAPL source, in particular to the down‐gradient area of the plume, is possible but could not be unequivocally confirmed. The time series provided direct evidence for MTBE biodegradation, but also a valuable insight on the sources of TBA.

Exceptionally Long MTBE Plumes of the Past Have Greatly Diminished.

Studies published in the late 1990s and early 2000s identified the presence of exceptionally long methyl tert-butyl ether (MTBE) plumes (more than 600 m or 2000 feet) in groundwater and have been cited in technical literature as characteristic of MTBE plumes. However, the scientific literature is incomplete in regard to the subsequent behavior and fate of these MTBE plumes over the past decade. To address this gap, this issue paper compiles recent groundwater monitoring records for nine exceptional plumes that were identified in prior studies. These nine sites exhibited maximum historical MTBE groundwater plume lengths ranging from 820 m (2700 feet) to 3200 m (10,500 feet) in length, exceeding the lengths of 99% of MTBE plumes, as characterized in multiple surveys at underground storage tank sites across the United States. Groundwater monitoring data compiled in our review demonstrate that these MTBE plumes have decreased in length over the past decade, with five of the nine plumes exhibiting decreases of 75% or more compared to their historical maximum lengths. MTBE concentrations within these plumes have decreased by 93% to 100%, with two of the nine sites showing significant decreases (98% and 99%) such that the regulatory authority has subsequently designated the site as requiring no further action.

Review of Quantitative Surveys of the Length and Stability of MTBE, TBA, and Benzene Plumes in Groundwater at UST Sites

Quantitative information regarding the length and stability condition of groundwater plumes of benzene, methyl tert-butyl ether (MTBE), and tert-butyl alcohol (TBA) has been compiled from thousands of underground storage tank (UST) sites in the United States where gasoline fuel releases have occurred. This paper presents a review and summary of 13 published scientific surveys, of which 10 address benzene and/or MTBE plumes only, and 3 address benzene, MTBE, and TBA plumes. These data show the observed lengths of benzene and MTBE plumes to be relatively consistent among various regions and hydrogeologic settings, with median lengths at a delineation limit of 10 µg/L falling into relatively narrow ranges from 101 to 185 feet for benzene and 110 to 178 feet for MTBE. The observed statistical distributions of MTBE and benzene plumes show the two plume types to be of comparable lengths, with 90th percentile MTBE plume lengths moderately exceeding benzene plume lengths by 16% at a 10-µg/L delineation limit (400 feet vs. 345 feet) and 25% at a 5-µg/L delineation limit (530 feet vs. 425 feet). Stability analyses for benzene and MTBE plumes found 94 and 93% of these plumes, respectively, to be in a nonexpanding condition, and over 91% of individual monitoring wells to exhibit nonincreasing concentration trends. Three published studies addressing TBA found TBA plumes to be of comparable length to MTBE and benzene plumes, with 86% of wells in one study showing nonincreasing concentration trends.

Using DNA‐Stable Isotope Probing to Identify MTBE‐ and TBA‐Degrading Microorganisms in Contaminated Groundwater

Although the anaerobic biodegradation of methyl tert-butyl ether (MTBE) and tert-butyl alcohol (TBA) has been documented in the laboratory and the field, knowledge of the microorganisms and mechanisms involved is still lacking. In this study, DNA-stable isotope probing (SIP) was used to identify microorganisms involved in anaerobic fuel oxygenate biodegradation in a sulfate-reducing MTBE and TBA plume. Microorganisms were collected in the field using Bio-Sep® beads amended with 13C5-MTBE, 13C1-MTBE (only methoxy carbon labeled), or13C4-TBA. 13C-DNA and 12C-DNA extracted from the Bio-Sep beads were cloned and 16S rRNA gene sequences were used to identify the indigenous microorganisms involved in degrading the methoxy group of MTBE and the tert-butyl group of MTBE and TBA. Results indicated that microorganisms were actively degrading 13C-labeled MTBE and TBA in situ and the 13C was incorporated into their DNA. Several sequences related to known MTBE- and TBA-degraders in the Burkholderiales and the Sphingomonadales orders were detected in all three13C clone libraries and were likely to be primary degraders at the site. Sequences related to sulfate-reducing bacteria and iron-reducers, such as Geobacter and Geothrix, were only detected in the clone libraries where MTBE and TBA were fully labeled with 13C, suggesting that they were involved in processing carbon from the tert-butyl group. Sequences similar to the Pseudomonas genus predominated in the clone library where only the methoxy carbon of MTBE was labeled with 13C. It is likely that members of this genus were secondary degraders cross-feeding on 13C-labeled metabolites such as acetate.

Use of Long-Term Monitoring Data to Evaluate Benzene, MTBE, and TBA Plume Behavior in Groundwater at Retail Gasoline Sites

AbstractLong-term groundwater monitoring data for 48 retail gasoline sites were analyzed to define the characteristics of affected groundwater plumes containing benzene, methyl tert-butyl ether (MTBE), and tert-butyl alcohol (TBA). Results of this analysis were used to determine the observed range and statistical distribution of current plume lengths, plume stability conditions, constituent concentration trends and attenuation rates, and the remediation timeframe for this population of sites. The goal of this evaluation was to characterize plume behavior as observed across a variety of hydrogeologic settings, on the basis of detailed groundwater monitoring records, rather than to define the site-specific factors controlling plume behavior. The results indicate that MTBE plumes in groundwater underlying a majority of these underground storage tank sites that were monitored for five years or longer (1) have significantly diminished in concentration over time, (2) are comparable in length to benzene plumes, ...

Assessment of MTBE biodegradation in contaminated groundwater using 13C and 14C analysis: Field and laboratory microcosm studies

Radiolabelled assays and compound-specific stable isotope analysis (CSIA) were used to assess methyl tert-butyl ether (MTBE) biodegradation in an unleaded fuel plume in a UK chalk aquifer, both in the field and in laboratory microcosm experiments. The 14C-MTBE radiorespirometry studies demonstrated widespread potential for aerobic and anaerobic MTBE biodegradation in the aquifer. However, δ13C compositions of MTBE in groundwater samples from the plume showed no significant 13C enrichment that would indicate MTBE biodegradation at the field scale. Carbon isotope enrichment during MTBE biodegradation was assessed in the microcosms when dissolved O2 was not limiting, compared with low in situ concentrations (2 mg/L) in the aquifer, and in the absence of O2. The microcosm experiments showed ubiquitous potential for aerobic MTBE biodegradation in the aquifer within hundreds of days. Aerobic MTBE biodegradation in the microcosms produced an enrichment of 7‰ in the MTBE δ13C composition and an isotope enrichment factor (e) of −1.53‰ when dissolved O2 was not limiting. However, for the low dissolved O2 concentration of up to 2 mg/L that characterizes most of the MTBE plume fringe, aerobic MTBE biodegradation produced an enrichment of 0.5–0.7‰, corresponding to an e value of −0.22‰ to −0.24‰. No anaerobic MTBE biodegradation occurred under these experimental conditions. These results suggest the existence of a complex MTBE-biodegrading community in the aquifer, which may consist of different aerobic species competing for MTBE and dissolved O2. Under low O2 conditions, the lower fractionating species have been shown to govern overall MTBE C-isotope fractionation during biodegradation, confirming the results of previous laboratory experiments mixing pure cultures. This implies that significant aerobic MTBE biodegradation could occur under the low dissolved O2 concentration that typifies the reactive fringe zone of MTBE plumes, without producing detectable changes in the MTBE δ13C composition. This observed insensitivity of C isotope enrichment to MTBE biodegradation could lead to significant underestimation of aerobic MTBE biodegradation at field scale, with an unnecessarily pessimistic performance assessment for natural attenuation. Site-specific C isotope enrichment factors are, therefore, required to reliably quantify MTBE biodegradation, which may limit CSIA as a tool for the in situ assessment of MTBE biodegradation in groundwater using only C isotopes.

Compound-Specific Stable Carbon Isotope Analysis of MTBE in Groundwater Contamination Fingerprinting Studies: The Use of Hydrogeologic Principles to Assess Its Validity

Methyl-tert-butyl ether (MTBE) is an oxygenate added to gasoline in the United States and elsewhere to boost the octane ratio and reduce tailpipe emissions. Because of its known usage time frame, the presence of MTBE can help estimate the age of gasoline releases to the environment. However, because of its ubiquity, its presence can no longer be used to fingerprint, or differentiate, gasoline plumes in groundwater. Compound-specific stable carbon isotope analysis has recently become a method for differentiating MTBE sources and plumes affecting groundwater. In the present study, groundwater samples were collected from 20 monitoring wells located at two gasoline service stations in central New Jersey in an attempt to differentiate the dissolved gasoline plumes and possibly identify their sources. The carbon isotope ratio (13C/12C expressed as δ13C) values for each sample were determined through gas chromatography–isotope ratio–mass spectrometry (GCIRMS), and the impacted area for each plume was delineated. As biological degradation of MTBE proceeds, δ13C becomes heavier; petroleum-degrading bacteria preferentially assimilate the lighter 12C isotope, thereby enriching the residual MTBE in the heavier 13C isotope. The isotopic composition of MTBE at different locations demonstrated the varying degree to which MTBE had been altered by biodegradation. The δ13C values can also provide insight into the type of release and the magnitude of biodegradation. Two types of leaks occurred at the sites: a shallow piping leak and deeper leak from a tank system. δ13C values in the vicinity of the shallow leak were heavier, revealing increased biodegradation in the soil, while δ13C values in the vicinity of the tank leak were lighter, revealing the lack of biological degradation in non-aqueous phase liquid (NAPL) trapped beneath the water table. An assessment is provided where the physical site characteristics responsible for petroleum weathering are considered in comparison with the measured δ13C values.

2D-cell experiment on methyl tert-butyl ether transport in saturated zone of groundwater

As an additive of gasoline, methyl tert-butyl ether (MTBE) has a higher solubility in water, which is about 20 times as high as that of benzene. This characteristic results in MTBE dissolving out of the gasoline into the soil and groundwater. Due to relative unique physicochemical behavior of MTBE it would be an ideal candidate for use in environmental forensic investigations. In order to study the transport and distribution of MTBE in saturated zone of ground water, a two-dimensional experimental cell was setup to simulate the real environment of the groundwater flow. The effects of soil and groundwater flow velocity on the MTBE transport were investigated. The results show that the mobile distance of MTBE in vertical direction was smaller than that in horizontal direction paralleling with the groundwater flow. Because the main dynamics of groundwater flow direction was convection and dispersion, the movement of MTBE is also diffusion in the vertical direction. In addition, the transport of MTBE was more quick in high permeability porous media, and the increase of groundwater flow velocity can accelerate the MTBE plume development, but the irregularity and randomness of the plume are enhanced synchronously. These research results can give some helps for the investigation of MTBE movement in the groundwater, also can make some references for other petroleum contamination behavior.

Comparison of MTBE concentrations in groundwater of urban and nonurban areas in Germany

The occurrence of the gasoline oxygenate methyl tert-butyl ether (MTBE) in groundwater samples from known fuel-contaminated sites ( n=29 samples), nonurban ( n=74) and urban sites ( n=67) in Germany was investigated. The analyses revealed detection frequencies of 58% (contaminated sites), 24% (nonurban sites) and 63% (urban sites) at a detection limit of 0.01 μg L −1. Median (maximum) MTBE concentrations were calculated for nonurban and urban samples as 0.18 μg L −1 (2.2 μg L −1) and 0.06 μg L −1 (48 μg L −1). The data from nonurban samples revealed MTBE detections mainly at public supply wells with higher pumping rates than monitoring wells. MTBE was more frequently detected in urban samples, most probably due to the higher atmospheric input and direct liquid emissions from motorways or gas stations. Higher concentrations above 1.0 μg L −1 in urban areas were found in wells located at industrial sites, where also a MTBE plume was accidentally detected during the study. The prevalence of MTBE in shallow aquifers was comparable to those in the USA.

Design of an MTBE Remediation Technology Evaluation

AbstractThis study examines the intrinsic variability of dissolved methyl tert‐butyl ether (MTBE) concentrations in ground water during the course of a pilot‐scale bioremedial technology trial in Port Hueneme, California. A pretrial natural gradient tracer experiment using bromide was conducted in an anaerobic test section of the aquifer to characterize hydrogeology. The results showed the presence of a complex velocity field in terms of vertical stratification and preferential flowpaths. The hydraulic conductivity at the test area varied by >2 orders of magnitude, and the effects of vertical stratification were made apparent by the tracers’ detection pattern, which was predominately higher in the lower part of the aquifer. Since historically the lower portion of the aquifer significantly influenced MTBE transport, it was emphasized by increasing the sampling frequency for MTBE and tracers during the pilot test that involved the intermittent addition of oxygen and propane into the aquifer. A second tracer experiment using bromide and deuterated MTBE (2H12‐MTBE) was conducted at the onset of the technology trial and after the aquifer was made aerobic. The continuous metering of the tracer solutions into the test area was maintained for 300 d. The results showed that 2H12‐MTBE behaved as a conservative tracer since (1) its concentrations increased throughout the study approaching its designed injected level and (2) the pattern of its detection resembled that of bromide. On the other hand, 2H12‐MTBE, which was purposefully introduced into the aquifer, behaved differently from that of the existing dissolved MTBE plume that emanated from a non–aqueous phase liquid (NAPL) source over a decade ago, thereby undergoing years of diffusion. The data imply that a detailed understanding of the complexity of the flow field was not possible by observing the intrinsic MTBE data alone.

Tracking Methyl tert-Butyl Ether in Groundwater: Four Years Later

Methyl tert-butyl ether (MtBE) has been used as a gasoline additive to increase the octane rating and reduce air pollution in the United States. Due to its high solubility in water, mobility, and low natural biodegradation potential in the subsurface environment, MtBE has become a significant groundwater contaminant. During the last decade, the public, scientific, and regulatory communities have gained a great deal of knowledge about MtBE behavior in the subsurface. This article presents a review of MtBE historical data and its trends over a four-year period at a number of gasoline impacted sites in Los Angeles, California. This article uses field data obtained in the last four years to compare the highest concentration of MtBE at a well with the current concentration at the same well, and with the current highest concentration at the site (may not be the same well as that with the highest historical MtBE value). Statistics of the MtBE data showed that the MtBE historical highest concentrations have decreased between one and two orders of magnitude over the last four years. This result may be explained by the active cleanup at the sites, natural attenuation, and a better management of MtBE. However, the correlation coefficient revealed that a site with a relatively high historical highest concentration (CHH) may not have a relatively high current concentration (CC) today at the same well, suggesting that once the source is removed, the center of the MtBE plume may migrate downgradient of groundwater flow. The correlation coefficient also showed that a site with the relatively high CHH relates to current highest concentrations (CCH) at the site.

Anaerobic Biodegradation of MTBE at a Gasoline Spill Site

AbstractTo manage risk or to implement natural attenuation as a remedy, regulatory agencies must understand the processes that attenuate methyl‐tert‐butyl ether (MTBE) in ground water. Most case studies and laboratory studies in the literature indicate that natural biodegradation is not important; however, recent reports indicate that natural biodegradation of MTBE plays an important role under certain conditions. In an MTBE plume at a retail gasoline station in Parsippany, New Jersey, the long‐term monitoring data indicated that the concentration of MTBE was slowly declining over time in the wells that were within the footprint of the plume. The ratio of tert‐butyl alcohol (TBA) to MTBE increased with distance from the source area, and the ratio of TBA to MTBE in individual monitoring wells in the plume increased over time. This anecdotal evidence of natural biodegradation of MTBE to TBA at field scale was confirmed with a microcosm study. Core material from the interior of the plume was used to construct the microcosms. Following an initial lag period of 58 d, the concentration of MTBE decreased from more than 1460 μg/L to less than 10 μg/L within 199 d of incubation. As concentrations of MTBE declined in the microcosms, concentrations of TBA increased. The decrease in concentration of MTBE in the microcosms could be accounted for by an increase in the concentration of TBA.

Using Direct‐Push Tools to Map Hydrostratigraphy and Predict MTBE Plume Diving

AbstractAt a number of sites, a plume of methyl tert‐butyl ether (MTBE) in ground water has dived below the screen of conventional monitoring wells and escaped detection. Techniques are needed to predict the vertical extent of MTBE in an aquifer. Two techniques that are emerging in the site characterization market are electrical conductivity logging and pneumatic slug testing performed in temporary push wells. These techniques were evaluated at a diving plume of MTBE in the aquifer that supplies water to the village of East Alton, Illinois. The plume stayed near the water table for the first 100 m from the potential sources and then dived below conventional monitoring over the next 100 m. At the location where the MTBE plume dived, the depth to water was 9.1 m below land surface. The first 10 m of material below the water table had an electrical conductivity near 100 mS/m, indicating silts and clays. An electrical conductivity near 25 mS/m, indicating sands or gravels, was encountered at a depth of 10.6 m below the water table, and the sands and gravel extended to a depth of at least 15.2 m below the water table. Pneumatic slug tests measured low hydraulic conductivity in the interval of silt and clay (0.34 and 0.012 m/d) and higher hydraulic conductivity in the interval with sands and gravels (12.5, 11.6, and 11.3 m/d). Ground water with the highest concentration of MTBE was produced just below the contact between the silt and clay and the sands and gravel.

Natural attenuation of MTBE at two petroleum-hydrocarbon spill sites

Methyl tert-butyl ether (MTBE) has been used as a gasoline additive to improve the combustion efficiency and to replace lead since 1978. Because it is widely used and it has been disposed inappropriately, MTBE has become a prevalent groundwater contaminant worldwide. In this study, two petroleum-hydrocarbon contaminated sites (Sites A and B) were selected to evaluate the occurrence and effectiveness of natural attenuation of MTBE at these two sites. Field investigation results indicate that the natural attenuation mechanisms of MTBE at both sites were occurring with the first-order attenuation rates of 0.0021 and 0.0048 1 day −1 at Sites A and B, respectively. Results also reveal that the intrinsic biodegradation pattern was the most important mechanism among the natural attenuation processes at both sites. Results from BIOSCREEN simulation suggest that biodegradation was responsible for 78 and 59% of MTBE mass reduction at Sites A and B, respectively. Investigation results show that MTBE plume at Site B could be effectively controlled via natural attenuation processes. However, MTBE plume at Site A has migrated to a farther downgradient area and passed the boundary line of the site. Thus, more active groundwater remedial technologies should be applied at Site A to protect the downgradient environment. Results from this study suggest that natural attenuation might be feasible to be used as a remedial option for the remediation of MTBE-contaminated site on the premise that (1) detailed site characterization has been conducted and (2) the occurrence and effectiveness of natural attenuation processes have been confirmed.

Determination of Henry’s law constant for methyl tert-butyl ether (MTBE) at groundwater temperatures

The dimensionless Henry’s law constant was determined for methyl tert-butyl ether (MTBE) at six temperatures (3, 5, 10, 15, 20 and 25 °C) by using a thermostatted flask (430 ml) containing an aqueous MTBE solution. The ratio between the gas phase and the water phase in the flask was approximately 1.7:1. The aim of this study was to acquire data needed to model the behaviour of MTBE at groundwater conditions. The dimensionless Henry’s law constant at 10 °C is approximately 0.01 but is 0.03 at 25 °C. This is important for modelling MTBE because the variation of vaporization cannot be disregarded at groundwater temperatures. In a second experiment the water solubility of MTBE was determined to be 62.1 g/l at 5 °C and 35.5 g/l at 20 °C). The high solubility at low temperatures could cause MTBE plumes from spills (fuel accidents etc.) to spread rapidly.

Review of Natural Attenuation of BTEX and MTBE in Groundwater

Monitored natural attenuation has become accepted as an appropriate “technology” for treating petroleum hydrocarbon-contaminated groundwater. Commonly, the monoaromatic compounds benzene, toluene, ethylbenzene, and xylenes (BTEX) and oxygenated additives, e.g., methyl tert-butyl ether (MTBE), are the major constituents of regulatory importance. Although a variety of naturally occurring attenuation mechanisms may reduce BTEX and MTBE concentrations, intrinsic biodegradation is the primary destructive mechanism. Attenuation by anaerobic biodegradation is important for BTEX compounds because of the relative abundance of anaerobic electron acceptors as compared with dissolved oxygen. Field studies indicate that MTBE is also biodegradable in shallow aquifers, but at a slower rate relative to BTEX compounds; thus, dispersion and dilution may also be important MTBE attenuation mechanisms. To demonstrate that natural attenuation is occurring, it is critical to document the proposed natural attenuation processes in the field. These processes often cause measurable “footprints.” At a large number of petroleum-hydrocarbon release sites, natural attenuation processes have been observed and documented and found to control the extent of migration of BTEX contaminants. Although the presence of MTBE may compromise the use of natural attenuation at some sites due to the greater mobility and persistence of MTBE compared to BTEX, some data indicate that MTBE plumes do eventually stabilize.

Estimation of Methyl tert -Butyl Ether Plume Length Using the Domenico Analytical Model

Methyl tert -butyl ether (MTBE) plume is controlled by many factors, primarily by groundwater flow velocity, dispersion, natural attenuation. This study employed an analytical model introduced by 3, J. Hydrol. 91, 49–58.) to describe the MTBE concentration distribution horizontal pattern and estimated the MTBE plume length. The model was applied to 90 leaking underground storage tank cases in Los Angeles, CA, U.S.A. The analytical model was calibrated with field data for each case using a Microsoft Excel spreadsheet program. Methyl tert -butyl ether concentrations in one source monitoring well and one to two downgradient centerline monitoring wells were used for each case study. When the centerline well is not available, the closest off-centerline wells were projected to the centerline using an ellipse trigonometry method. The model parameter values for longitudinal dispersivity, groundwater velocity, and degradation rate constant were calibrated using the field data and then used to estimate the maximum distance between source well and the plume edge. This study demonstrates that the Domenico model can be applied to MTBE plume investigation when adequate field data are available. The correlation coefficients calculated based on the results of the 90 case studies indicate that MTBE plume length has a poor correlation with MTBE concentration at the source well, and a moderate negative correlation with the degradation rate constant (−0.65) and λ/ν ratio (0.64). Furthermore, MTBE plume length has a poor correlation with the longitudinal dispersivity (−0.4), hydraulic gradient (−0.1), and groundwater velocity (0.17).

Estimation of Methyl tert -Butyl Ether Plume Length Using the Domenico Analytical Model

Methyl tert -butyl ether (MTBE) plume is controlled by many factors, primarily by groundwater flow velocity, dispersion, natural attenuation. This study employed an analytical model introduced by Domemico (1987, J. Hydrol 91 , 49-58.) to describe the MTBE concentration distribution horizontal pattern and estimated the MTBE plume length. The model was applied to 90 leaking underground storage tank cases in Los Angeles, CA, U.S.A. The analytical model was calibrated with field data for each ease using a Microsoft Excel spreadsheet program. Methyl tert -butyl ether concentrations in one source monitoring well and one to two downgradient centerline monitoring wells were used for each case study. When the centerline well is not available, the closest off-centerline wells were projected to the centerline using an ellipse trigonometry method. The model parameter values for longitudinal dispersivity, groundwater velocity, and degradation rate constant were calibrated using the field data and then used to estimate the maximum distance between source well and the plume edge. This study demonstrates that the Domenico model can be applied to MTBE plume investigation when adequate field data are available. The correlation coefficients calculated based on the results of the 90 case studies indicate that MTBE plume length has a poor correlation with MTBE concentration at the source well, and a moderate negative correlation with the degradation rate constant ( m 0.65) and u / v ratio (0.64). Furthermore, MTBE plume length has a poor correlation with the longitudinal dispersivity ( m 0.4), hydraulic gradient ( m 0.1), and groundwater velocity (0.17).

In situ MTBE biodegradation supported by diffusive oxygen release.

Microcosm studies with sediments from Vandenberg Air Force Base, CA, suggest that native aerobic methyl tert-butyl ether (MTBE)-degrading microorganisms can be stimulated to degrade MTBE. In a series of field experiments, dissolved oxygen has been released into the anaerobic MTBE plume by diffusion through the walls of oxygen-pressurized polymeric tubing placed in contact with the flowing groundwater. MTBE concentrations were decreased from several hundred to less than 10 microg/L during passage through the induced aerobic zone, due apparently to in situ biodegradation: abiotic MTBE loss mechanisms were insignificant. Lag time for initiation of degradation was less than 2 months, and the apparent pseudo-first-order degradation rate was 5.3 day(-1). Additional MTBE was added in steps to raise the influent concentration to a maximum of 2.1 mg/L. With each step, MTBE was degraded within the preestablished aerobic treatment zone at rates ranging from 4.4 to 8.6 day(-1). Excess dissolved oxygen suggested that even higher MTBE concentrations could have been treated. Continued flow through the treatment zone was repeatedly confirmed through tracer and other tests. These and others' results suggest that it is possible to create permeable in situ treatment zones solely by releasing oxygen to support native microbial degradation of MTBE.

Lead Isotopes as Age-Sensitive, Genetic Markers in Hydrocarbons: 1. Copartitioning of Lead with MTBE into Water and Implications for MTBE-Source Correlations

The potential of lead isotopes as tracers of methyl tert-butyl ether (MTBE) in groundwater was assessed in a study that included: an evaluation of organolead’s chemical affinity with individual gasoline hydrocarbon constituents; gasoline/water partitioning experiments; and site-specific investigations of MTBE- impacted groundwater. Concentrations of organolead were correlated with concentrations of MTBE, pentane, benzene, toluene, xylenes, ethylbenzene, 1,2,4-trimethylbenzene, and methyl- cyclohexane. Significant correlations (R2 > 0.98) were observed between organolead and methylcyclohexane, MTBE, and benzene; the latter two being the most water-soluble gasoline constituents. To evaluate the possible role of MTBE in the transport of organolead from a product into the aqueous phase, two unleaded gasoline–aqueous partitioning experiments were performed. Each unleaded gasoline was sequentially extracted with fresh volumes of water over a period of ~3 weeks; a strong systematic relationship (R2 = 0.998) was observed between lead and MTBE in the aqueous phase, indicating that the organo- lead–MTBE chemical affinity in unleaded gasoline was maintained as these gasoline constituents partitioned into water. The lead isotopic equilibrium between unleaded gasoline and aqueous- phase lead also supports this conclusion. Site-specific analyses of groundwater impacted by MTBE corroborate the laboratory results, showing that groundwater impacted by MTBE releases exhibit lead isotopic ratios identical to those of unleaded gasoline responsible for the release. Our results also suggest that the fate/transport of MTBE plumes can be monitored by integrated lead/MTBE data, possibly to distances exceeding 500 m from the source of the release.