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

At 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.     

MTBE in Drinking Water Production – Occurrence and Efficiency of Treatment Technologies

In Germany, the gasoline additive methyl tert‐butyl ether (MTBE) is almost constantly detected in measurable concentrations in surface waters and is not significantly removed during riverbank filtration. The removal of MTBE from water has been the focus of many studies that mostly were performed at high concentration levels and centred in understanding the mechanisms of elimination. In order to assess the performance of conventional and advanced water treatment technologies for MTBE removal in the low concentration range further studies were undertaken. Laboratory experiments included aeration, granulated activated carbon (GAC) adsorption, ozonation and advanced oxidation processes (AOP). The results show that the removal of MTBE by conventional technologies is not easily achieved. MTBE is only removed by aeration at high expense. Ozonation at neutral pH values did not prove to be effective in eliminating MTBE at all. The use of ozone/H₂O₂ (AOP) may lead to a partly elimination of MTBE. However, the ozone/H₂O₂ concentrations required for a complete removal of MTBE from natural waters is much higher than the ozone levels applied nowadays in waterworks. MTBE is only poorly adsorbed on activated carbon, thus GAC filtration is not efficient in eliminating MTBE. A comparison with real‐life data from German waterworks reveals that if MTBE is detected in the raw water it is most often found in the corresponding drinking water as well due to the poor removal efficiency of conventional treatment steps.     

Progress in Remediation of Groundwater at Petroleum Sites in California

Quantifying the overall progress in remediation of contaminated groundwater has been a significant challenge. We utilized the GeoTracker database to evaluate the progress in groundwater remediation from 2001 to 2011 at over 12,000 sites in California with contaminated groundwater. This paper presents an analysis of analytical results from over 2.1 million groundwater samples representing at least $100 million in laboratory analytical costs. Overall, the evaluation of monitoring data shows a large decrease in groundwater concentrations of gasoline constituents. For benzene, half of the sites showed a decrease in concentration of 85% or more. For methyl tert‐butyl ether (MTBE), this decrease was 96% and for TBE, 87%. At remediation sites in California, the median source attenuation rate was 0.18/year for benzene and 0.36/year for MTBE, corresponding to half‐lives of 3.9 and 1.9 years, respectively. Attenuation rates were positive (i.e., decreasing concentration) for benzene at 76% of sites and for MTBE at 85% of sites. An evaluation of sites with active remediation technologies suggests differences in technology effectiveness. The median attenuation rates for benzene and MTBE are higher at sites with soil vapor extraction or air sparging compared with sites without these technologies. In contrast, there was little difference in attenuation rates at sites with or without soil excavation, dual phase extraction, or in situ enhanced biodegradation. The evaluation of remediation technologies, however, did not evaluate whether specific systems were well designed or implemented and did not control for potential differences in other site factors, such as soil type.     

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), or ¹³C₄‐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 three ¹³C 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.     

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.     

Degradation of Six Selected Ultraviolet Filters in Aquifer Materials Under Various Redox Conditions

Laboratory biodegradation batch studies were performed to investigate the degradation behavior of six selected UV filters, namely benzophenone‐3 (BP‐3), 3‐(4‐methylbenzylidene) camphor (4‐MBC), Octyl 4‐methoxycinnamate (OMC), Octocrylene (OC), 2‐(3‐t‐butyl‐2‐hydroxy‐5‐methylphenyl)‐5‐chloro benzotriazole (UV‐326), and 2‐(2’‐hydroxy‐5’‐octylphenyl)‐benzotriazole (UV‐329) in an aquifer microcosm (groundwater and aquifer sediment mixture) under aerobic and anaerobic (nitrate, sulfate, and Fe(III) reducing) conditions within 77 d. The results from the biodegradation experiments showed that the six UV filters were degraded well in the aquifer materials under different redox conditions. Rapid biodegradation was observed for BP‐3 and OMC in the aquifer materials, with their half‐lives of 1.5‐8.8 d and 1.3‐5.2 d, respectively. In most cases, aerobic conditions were more favorable for the degradation of the UV filters in aquifer materials. Relatively slow degradation of 4‐MBC, UV‐326, and UV‐329 under anaerobic conditions was noted with their half‐lives ranging between 47 d and 126 d, indicating potential persistence in anaerobic aquifers. The results showed that redox conditions could have significant effects on biodegradation of the UV filters in aquifers.     

Potential Occurrence of MTBE and BTEX in Groundwater Resources of Amman–Zarqa Basin,Jordan

This study investigates potential occurrence, distribution, and sources of the newly added gasoline oxygenate, methyl‐tert‐butyl ether (MTBE) and the petroleum derivatives benzene, toluene, ethylbenzene, and xylenes called collectively, BTEX, in Jordan's heavily populated Amman–Zarqa Basin (AZB). It presents the first data on the levels of MTBE and BTEX in the aquifers of this basin. One hundred and seventy‐nine (179) groundwater wells were sampled near petrol service stations, oil refinery storage tanks, car wrecks, bus stations, and chemical industries at different locations in the basin. Headspace GC and purge and trap GC–MS were utilized to determine the target substances in the samples. Concentrations of BTEX varied between no‐detection (minimum) for all of them to 6.6 µg/L (maximum) for ethylbenzene. MTBE was found in few samples but none has exceeded the regulated levels; its concentrations ranged between no‐detection to 4.1 µg/L. However, though the contamination levels are very low they should be considered alarming.     

An Empirical Evaluation of the Influence of Ethanol on Natural Attenuation of Gasoline Constituents

Since the 1990s, questions have arisen as to whether the release of ethanol‐blended fuel will inhibit natural attenuation of other gasoline constituents in groundwater. This study evaluated the hypothesis that ethanol affects hydrocarbon attenuation and whether the use of ethanol‐blended fuel alters the applicability of monitored natural attenuation (MNA) as an approach for managing risks at fuel‐release sites. Groundwater data from California's GeoTracker database were used to compare attenuation of benzene, toluene, methyl tert‐butyl ether (MTBE), and tert‐butyl alcohol (TBA) at sites with and without detections of ethanol. Excel‐based tools were developed to conduct attenuation evaluations on thousands of wells simultaneously. Ethanol was detected at least once in 4.5% of the wells and 0.6% of the samples of which it was analyzed. The distribution of Mann‐Kendall concentration trend analysis results and first‐order attenuation rates were essentially the same at sites with or without ethanol detections. Median plume lengths were shorter at sites where ethanol had not been detected compared to sites where ethanol was detected (36 vs. 43 m for benzene; 36 vs. 42 m for toluene; 43 vs. 52 m for MTBE; and 44 vs. 59 m for TBA). However, the distribution of plume lengths was similar irrespective of ethanol concentrations, suggesting other factors may influence plume elongation. Finally, while anaerobic ethanol degradation can result in methane generation, the distributions of methane concentrations were the same at sites with and without ethanol detections. These results suggest that the use of ethanol‐blended fuel should not limit the application of MNA at most biodegrading fuel‐release sites.     

Removal of Volatile Organic Compounds in Vertical Flow Filters: Predictions from Reactive Transport Modeling

Vertical flow filters are containers filled with porous medium that are recharged from top and drained at the bottom, and are operated at partly saturated conditions. They have recently been suggested as treatment technology for groundwater containing volatile organic compounds (VOCs). Numerical reactive transport simulations were performed to investigate the relevance of different filter operation modes on biodegradation and/or volatilization of the contaminants and to evaluate the potential limitation of such remediation mean due to volatile emissions. On the basis of the data from a pilot‐scale vertical flow filter intermittently fed with domestic waste water, model predictions on the system’s performance for the treatment of contaminated groundwater were derived. These simulations considered the transport and aerobic degradation of ammonium and two VOCs, benzene and methyl tertiary butyl ether (MTBE). In addition, the advective‐diffusive gas‐phase transport of volatile compounds as well as oxygen was simulated. Model predictions addressed the influence of depth and frequency of the intermittent groundwater injection, degradation rate kinetics, and the composition of the filter material. Simulation results show that for unfavorable operation conditions significant VOC emissions have to be considered and that operation modes limiting VOC emissions may limit aerobic biodegradation. However, a suitable combination of injection depth and composition of the filter material does facilitate high biodegradation rates while only little VOC emissions take place. Using such optimized operation modes would allow using vertical flow filter systems as remediation technology suitable for groundwater contaminated with volatile compounds.     

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.     

MTBE, TBA, and TAME Attenuation in Diverse Hyporheic Zones

Groundwater contamination by fuel-related compounds such as the fuel oxygenates methyl tert -butyl ether (MTBE), tert -butyl alcohol (TBA), and tert -amyl methyl ether (TAME) presents a significant issue to managers and consumers of groundwater and surface water that receives groundwater discharge. Four sites were investigated on Long Island, New York, characterized by groundwater contaminated with gasoline and fuel oxygenates that ultimately discharge to fresh, brackish, or saline surface water. For each site, contaminated groundwater discharge zones were delineated using pore water geochemistry data from 15 feet (4.5 m) beneath the bottom of the surface water body in the hyporheic zone and seepage-meter tests were conducted to measure discharge rates. These data when combined indicate that MTBE, TBA, and TAME concentrations in groundwater discharge in a 5-foot (1.5-m) thick section of the hyporheic zone were attenuated between 34% and 95%, in contrast to immeasurable attenuation in the shallow aquifer during contaminant transport between 0.1 and 1.5 miles (0.1 to 2.4 km). The attenuation observed in the hyporheic zone occurred primarily by physical processes such as mixing of groundwater and surface water. Biodegradation also occurred as confirmed in laboratory microcosms by the mineralization of U- ¹⁴C-MTBE and U-¹⁴C-TBA to ¹⁴CO₂ and the novel biodegradation of U- ¹⁴C-TAME to ¹⁴CO₂ under oxic and anoxic conditions. The implication of fuel oxygenate attenuation observed in diverse hyporheic zones suggests an assessment of the hyporheic zone attenuation potential (HZAP) merits inclusion as part of site assessment strategies associated with monitored or engineered attenuation.     

A Practical Approach to the Design, Monitoring, and Optimization of In Situ MTBE Aerobic Biobarriers

A paradigm for the design, monitoring, and optimization of in situ methyl tert -butyl ether (MTBE) aerobic biobarriers is presented. In this technology, an oxygen-rich biologically reactive treatment zone (the "biobarrier") is established in situ and downgradient of the source of dissolved MTBE contamination in groundwater, typically gasoline-impacted soils resulting from leaks and spills at service station sites or other fuel storage and distribution facilities. The system is designed so that groundwater containing dissolved MTBE flows to, and through, the biobarrier treatment zone, ideally under natural gradient conditions so that no pumping is necessary. As the groundwater passes through the biobarrier, the MTBE is converted by microorganisms to innocuous by-products. The system also reduces concentrations of other aerobically degradable chemicals dissolved in the groundwater, such as benzene, toluene, xylenes, and tert -butyl alcohol. This design paradigm is based on experience gained while designing, monitoring, and optimizing pilot-scale and full-scale MTBE biobarrier systems. It is largely empirically based, although the design approach does rely on simple engineering calculations. The paradigm emphasizes gas injection–based oxygen delivery schemes, although many of the steps would be common to other methods of delivering oxygen to aquifers.     

Evaluation of the Effectiveness of Different Methods for the Remediation of Contaminated Groundwater by Determining the Petroleum Hydrocarbon Content

The effectiveness of different remediation procedures for decreasing the amount of TPH (total petroleum hydrocarbons) in contaminated groundwater was evaluated at the site of a former refinery. The investigations were carried out on samples taken from several gravel based HSSF (horizontal subsurface flow) constructed wetlands (CW) which differed in relation to their filter material additives (no additive, charcoal, and ferric oxides additives) and examined the potential effect of these additives on the overall treatment efficiency. Samples of the following gravel based HSSF CW were investigated. No filter additive (system A), 0.1% activated carbon (system B), 0.5% iron(III) hydroxide (system C), and the reference (system D). Systems A–C were planted with common reed (Phragmites australis), whereas system D remained unplanted. In addition, the influence of seasonal conditions on the reduction of these hydrocarbons and the correlation between the amounts of TPH and BTEX (benzene, toluene, ethylbenzene, and xylene isomers), on the one hand, and methyl tert‐butyl ether, on the other, was investigated. The study was carried out by using a modified GC‐FID approach and multivariate methods. The investigations carried out in the first year of operation demonstrated that the effectiveness of the petroleum hydrocarbon removal was highest and reached a level of 93 ± 3.5% when HSSF filters with activated carbon as a filter additive were used. This remediation method allowed the petroleum hydrocarbon content to be reduced independently of seasonal conditions. The correlation between the reduction of TPH and BTEX was found to be R = 0.8824. Using this correlation coefficient, the time‐consuming determination of the BTEX content was no longer necessary.     

Degradation of organic matter from wastewater using advanced primary treatment by O3 and O3/UV in a pilot plant

The oxidation of organic matter from wastewater using ozone, ultraviolet radiation and ozone/UV oxidation was evaluated in a pilot plant, applying a continuous effluent arising from the Autonomous Metropolitan University wastewater treatment plant. The oxidation was measured as the efficiency to remove organic load, measured as chemical oxygen demand. The use of ozone and UV was evaluated separately and in combination through a continuous process. Three different ozone doses (0.6–1.2 mg O₃/L) and three different UV radiation fluencies (6.7–20.12 mJ/cm²) were assessed. A synergistic effect of the combined process ozone/UV was demonstrated, and a maximal chemical oxygen demand reduction was achieved both processes. Due to residence times used (less than 1 min), 36% of chemical oxygen demand reduction was obtained when ozone treatment was evaluate separately and only 9% using ultraviolet radiation.     

A coupled model for simulating surface water and groundwater interactions in coastal wetlands

Coastal wetlands are characterized by strong, dynamic interactions between surface water and groundwater. This paper presents a coupled model that simulates interacting surface water and groundwater flow and solute transport processes in these wetlands. The coupled model is based on two existing (sub) models for surface water and groundwater, respectively: ELCIRC (a three‐dimensional (3‐D) finite‐volume/finite‐difference model for simulating shallow water flow and solute transport in rivers, estuaries and coastal seas) and SUTRA (a 3‐D finite‐element/finite‐difference model for simulating variably saturated, variable‐density fluid flow and solute transport in porous media). Both submodels, using compatible unstructured meshes, are coupled spatially at the common interface between the surface water and groundwater bodies. The surface water level and solute concentrations computed by the ELCIRC model are used to determine the boundary conditions of the SUTRA‐based groundwater model at the interface. In turn, the groundwater model provides water and solute fluxes as inputs for the continuity equations of surface water flow and solute transport to account for the mass exchange across the interface. Additionally, flux from the seepage face was routed instantaneously to the nearest surface water cell according to the local sediment surface slope. With an external coupling approach, these two submodels run in parallel using time steps of different sizes. The time step (Δt_g) for the groundwater model is set to be larger than that (Δt_s) used by the surface water model for computational efficiency: Δt_g = M × Δt_s where M is an integer greater than 1. Data exchange takes place between the two submodels through a common database at synchronized times (e.g. end of each Δt_g). The coupled model was validated against two previously reported experiments on surface water and groundwater interactions in coastal lagoons. The results suggest that the model represents well the interacting surface water and groundwater flow and solute transport processes in the lagoons. Copyright © 2011 John Wiley & Sons, Ltd.     

The Occurrence and Persistence of MTBE in Groundwater in Windham,Maine, USA

A study was conducted from July 1998 through November 2007 on the occurrence and distribution of the fuel oxygenate methyl tert-butyl ether (MTBE) in a large sand and gravel aquifer located in southern Maine. MTBE was detected in 44% of 129 water samples collected from monitoring wells in concentrations up to 38.7 µg/L (reporting limit = 0.1 µg/L). The number of wells with detectable quantities of MTBE declined slightly between 1999 and 2007, but in general MTBE persisted throughout the period of study. Overall, MTBE was detected more frequently in the shallow and more transmissive parts of the aquifer. There was a statistically significant difference (p < 0.001) for MTBE concentrations relative to nearby land uses. MTBE was detected in 83% of the samples collected from wells in low-density residential areas, in 50% of samples from urban areas, and in 60% of samples from undeveloped areas. The concentrations of MTBE in the test wells were compared across the sample dates for trends and seven wells had a positive trend (Mann–Kendall statistic), but none was significant at p < 0.05. Nine wells had a negative trend, but only one was significant at p < 0.05. Three wells had no trend. The absence of strong or even consistent trends indicates that MTBE persists in shallow groundwater, even after gasoline formulations were changed to reduce or eliminate MTBE.     

Degradation of Malathion and Parathion by Ozonation,Photolytic Ozonation,and Heterogeneous Catalytic Ozonation Processes

The oxidation of organophosphorus pesticides (OPPs), such as malathion and parathion, in aqueous solution was studied using conventional ozonation (O₃), photolytic ozonation (O₃/UV, O₃/UV/H₂O₂), and heterogeneous catalytic ozonation (O₃/TiO₂/UV) processes. Experiments were performed in batch mode at laboratory scale and processes were compared in terms of disappearance kinetics. The best results of pesticide mineralization were obtained when TiO₂ particles in combination with ozone (O₃) and UV photolysis (λ = 254 nm) were applied. Decomposition of 99% of parent compounds were achieved in 10 min and oxon derivatives were completely removed in 30 min. The initial reaction rate increases linearly with increasing catalyst amount. Toxicity measurements of the treated solutions were carried out in order to evaluate the efficiency of the treatment methods. No detoxification was achieved for O₃ and O₃/UV applications. Heterogeneous photocatalytic ozonation was shown to be feasible for achieving complete decomposition of OPPs and their oxon intermediates.     

Understanding the extent of interactions between groundwater and surface water through major ion chemistry and multivariate statistical techniques

The present work examines the possible use of major ion chemistry and multivariate statistical techniques as a rapid and relatively cost‐effective method of identifying the extent of groundwater and surface water (GW–SW) interaction in an urban setting. The original hydrogeochemical dataset consists of groundwater (n = 114), stream water (n = 42) and drain water (n = 24) samples, collected twice in a year for the pre‐ and post‐monsoon seasons, for three successive years along an 8 km reach of the Delhi segment of River Yamuna, India. The dynamic and similar seasonal changes of hydro‐geochemical facies and major ion trends of river, drain and groundwater samples indicate the existence of an empirical relationship between GW and SW. Results of both R‐ and Q‐mode factor and cluster analyses highlight multi‐scale control of the fluid exchange distributions, with distinct seasonal alteration in mode and extent of GW–SW interaction, namely, the influence of the mixing zones between urban river and groundwater and the pattern of groundwater flow through the river bed. Hierarchical cluster analysis (HCA) of sampling locations efficiently illustrates different groups that comprise samples severely influenced by contaminated surface water downstream and the upstream fresh water samples. These results substantiate the strong exchange processes between GW and SW all along the stretch. The study shows that the combination of an empirical and statistical relationship between different ionic species and sampling locations can provide greater confidence in identifying the extent of GW–SW interaction/exchange processes. Copyright © 2008 John Wiley & Sons, Ltd.     

Fate of MTBE Relative to Benzene in a Gasoline-Contaminated Aquifer (1993–98)

Methyl tert -butyl ether (MTBE) and benzene have been measured since 1993 in a shallow, sandy aquifer contaminated by a mid-1980s release of gasoline containing fuel oxygenates. In wells downgradient of the release area, MTBK was detected before benzene, reflecting a chromatographic-like separation of these compounds in the direction of ground water flow. Higher concentrations of MTBE and benzene were measured in the deeper sampling ports of multilevel sampling wells located near the release area, and also up to 10 feet (3 m) below the water table surface in nested wells located farther from the release area. This distribution of higher concentrations at depth is caused by recharge events that deflect originally horizontal ground water flowlines. In the laboratory, microcosms containing aquifer material incubated with uniformly labeled ¹⁴C-MTBE under aerobic and anaerobic. Fe(III)-reducing conditions indicated a low but measurable biodegradation potential (<3%¹⁴C-MTBW as ¹⁴CO₂) after a seven-month incubation period, Tert -butyl alcohol (TBA), a proposed microbial-MTBE transformation intermediate, was detected in MTBE-contaminated wells, but TBA was also measured in unsaturated release area sediments. This suggests that TBA may have been present in the original fuel spilled and does not necessarily reflect microbial degradation of MTBE. Combined, these data suggest that milligram per liter to microgram per liter decreases in MTBE concentrations relative to benzene are caused by the natural attenuation processes of dilution and dispersion with less-contaminated ground water in the direction of flow rather than biodegradation at this point source gasoline release site.     

Pump‐and‐Treat Groundwater Remediation Using Chlorine/Ultraviolet Advanced Oxidation Processes

Comparative studies of the use of chlorine/ultraviolet (Cl₂/UV) and hydrogen peroxide/ultraviolet (H₂O₂/UV) Advanced oxidation processes (AOPs) to remove trichloroethylene (TCE) from groundwater in a pump‐and‐treat application were conducted for the first time at the full‐scale operational level at two water treatment facilities in Northern California. In these studies, aqueous chlorine replaced hydrogen peroxide in the AOP treatment step, where the oxidant is exposed to UV light to produce highly reactive radical species that degrade groundwater contaminants. TCE removal rates as a function of initial chlorine dose and pH were then determined. At the site where the natural pH of the water was 7.1, TCE was removed (to a concentration of less than 0.5 µg/L) for nearly every chlorine dose point tested, and pH adjustment slightly enhanced the treatment process at this facility. The second site had a high natural pH of 7.7, and here, TCE was not completely removed for any chlorine dose up to 5.7 mg/L, although TCE removal did increase when the chlorine dose increased between 0.9 and 3.6 mg/L. Residual TCE remaining in the water post‐Cl₂/UV was readily removed using active carbon filtration, which is part of the overall treatment train at this facility. These studies also verified that Cl₂/UV AOP did not interfere with the photolysis of N‐nitrosodimethylamine or result in an effluent acutely toxic toward Ceriodaphnia dubia. Comparative economic analysis revealed that the chemical costs associated with Cl₂/UV AOP were 25 to 50% of the costs associated with in place H₂O₂/UV AOP treatment.