in situ remediation of mtbe utilizing ozone

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REMEDIATION Winter 2002

There has been a great deal of focus on methyl tertiary butyl ether (MTBE) over the past few

years by local, state, and federal government, industry, public stakeholders, the environmental

services market, and educational institutions. This focus is, in large part, the result of the

widespread detection of MTBE in groundwater and surface waters across the United States. The

presence of MTBE in groundwater has been attributed primarily to the release from

underground storage tank (UST) systems at gasoline service stations. MTBE’s physical and

chemical properties are different than other constituents of gasoline that have traditionally been

cause for concern [benzene, toluene, ethylbenzene, and xylenes (BTEX)]. This difference in

properties is why MTBE migrates differently in the subsurface environment and exhibits different

constraints relative to mitigation and remediation of MTBE once it has been released to

subsurface soils and groundwater.

Resource Control Corporation (RCC) has accomplished the remediation of MTBE from

subsurface soil and groundwater at multiple sites using ozone. RCC has successfully applied

ozone at several sites with different lithologies, geochemistry, and concentrations of constituents

of concern. This article presents results from several projects utilizing in situ chemical oxidation

with ozone. On these projects MTBE concentrations in groundwater were reduced to remedial

objectives usually sooner than anticipated. © 2002 Wiley Periodicals, Inc.

INTRODUCTION

Methyl tertiary butyl ether (MTBE) is a fuel additive that has been added to gasolineas an octane enhancing replacement for lead and more recently as an oxygenate, addedto reformulated gasoline to reduce air emissions.The more prevalent use of MTBE hasresulted in an increased presence of it in our environment.Various studies have beenand are being conducted to define the health effects of MTBE in the environment.

MTBE has received a great deal of attention and is being evaluated as anotherchemical in the environment reportedly causing public health problems.The United StatesEnvironmental Protection Agency (EPA) considers MTBE a possible human carcinogen,and it produces a disagreeable taste and odor at very low concentrations in drinking water.Based on these factors, the EPA has issued a drinking water advisory concentration of 20to 40 parts per billion (ppb) as a guideline for permissible levels in potable water.

MTBE was first associated with gasoline beginning in 1979, when the AtlanticRichfield Company was authorized by the EPA to add it to their gasolineformulation. MTBE was first used as an octane booster, to replace lead in gasoline.Initially, MTBE was used at relatively low concentrations (1 percent to 3 percent);gradually increasing to around 7 percent by the late 1980s.

© 2002 Wiley Periodicals, Inc.Published online in Wiley Interscience (www.interscience.wiley.com). DOI: 10.1002/rem.10055

Jeffrey C. Dey

Paul Rosenwinkel

Kevin Wheeler

In Situ Remediation of MTBE UtilizingOzone

The relatively highsolubility of MTBE, whichallows it to mix well withgasoline, also causes it todissolve in groundwater athigh concentrations.

MTBE is in a family of gasoline additives known as oxygenates, which improve thecombustion of gasoline in engines, and thus burn cleaner. As a consequence, MTBEconcentrations in unleaded gasoline increased, up to 15 percent, during the early 1990sin response to the requirements of the EPA’s Clean Air Act Amendments. However,while MTBE has been instrumental in improving the nation’s air quality, it has beendetrimental to groundwater and surface water quality.

Numerous reports and investigations presented over the past five years haveidentified widespread detection of MTBE in groundwater and surface water acrossthe country. According to the U.S. Geological Survey’s (USGS) National Water-Quality Assessment program in 1993 and 1994, MTBE was the second mostfrequently detected chemical in shallow groundwater from urban areas. Of the 210wells and springs sampled by the USGS, 27 percent contained MTBE. This widedistribution and occurrence of MTBE is due in large part to the same chemicalcharacteristics that make it well suited as a gasoline additive. The relatively highsolubility of MTBE, which allows it to mix well with gasoline, also causes it todissolve in groundwater at high concentrations. MTBE has a low adsorptioncoefficient that prevents it from significantly adsorbing to soil, which causes it totravel farther and faster through subsurface soils. The chemical stability of MTBE,which is a beneficial property for a fuel additive, causes it to persist for long periodsof time in the subsurface without degrading.

Overall, MTBE is undesirable in groundwater, even at low concentrations, becauseit spreads rapidly in water and is extremely persistent.Thus the question remains: whatis the best way to remediate MTBE?

MTBE REMEDIATION

The effectiveness of remediation techniques is directly affected by the physical andchemical nature of the constituent to be remediated. In general, MTBE is much lesslikely to adsorb to soil or organic carbon than other components of gasoline. MTBE isabout 30 times more soluble than benzene in water. MTBE’s Henry’s Law constant isabout one tenth that of benzene, and therefore, is much more difficult to remove fromthe dissolved phase.

MTBE behaves differently than other gasoline constituents of concern, and sitecharacterization plans and remedial designs should consider the compound’s specificbehaviors. Conventional groundwater remediation techniques including pump and treat,air sparging, bioremediation, and point-of-use treatment have been used to remediateMTBE in groundwater. Each of these remedial methods has shown some promise andhas site-specific cases where they have been quite effective for removing or remediatingMTBE. However, the consensus of the regulated community is that these traditionalmethods generally used to remediate BTEX components are typically more expensiveand lead to longer remediation projects. MTBE’s high solubility in water, low rate ofadsorption to soil, and low rate of biodegradation are some of the key characteristicsthat impact remediation.

OZONE

Ozone is a molecule comprised of three oxygen atoms. Ozone is a relatively unstablemolecule with a very short half-life.The half-life of ozone in air is approximately two

In Situ Remediation of MTBE Utilizing Ozone

© 2002 Wiley Periodicals, Inc.78

minutes, as it degrades back to oxygen. In water, dissolved ozone has a half-life ofapproximately 20 minutes, as it reacts with water to form free-radical ions, ordegrades back to oxygen.This short life span makes ozone ideal for environmentalremediation of contaminants, as it reacts quickly in the subsurface and rapidly revertsto harmless oxygen.

During its short life span, ozone is a very reactive oxidizer. Ozone has the thirdhighest oxidation potential in nature (behind fluorine and hydroxyl-radicals).Thehigher oxidation potential of ozone allows it to destroy a wide range of compounds,including MTBE.

Ozone can be introduced to the subsurface as either a gas (ozone sparging) or aliquid (dissolved ozone injection). In gas phase, ozone is typically introduced to thesubsurface (either alone or with air or oxygen) through specifically designed ozonesparge wells.The gas sparge rate is typically lower than that of air sparging, because theaim is to maximize mass transfer to the dissolved phase and/or maximize contact timewith contaminants. In aqueous phase, ozone dissolved in water is typically injected tothe subsurface through injection wells, trenches, or infiltration galleries.The result ofusing ozone for environmental cleanup is rapid remediation of some of the most arduouscontaminants, including MTBE.

When ozone is dissolved in groundwater, the groundwater itself becomes aremediating agent, which can destroy contaminants. Dissolved ozone distribution in thesaturated zone is typically better than gas distribution, as it is less affected bypreferential flow pathways, upward migration, and has the benefit of aqueous diffusion.

In groundwater, dissolved ozone can directly destroy MTBE, it can degrade to free-radical ions (which in turn oxidize MTBE), or it can degrade to dissolved oxygen, whichcan enhance natural biodegradation of residual contaminants. Ozone is up to 12.5 timesmore soluble in water than oxygen, which allows ozone to dissolve into water at morethan 500 ppm and saturate the water with dissolved oxygen following degradation.

The oxidation of MTBE occurs through the following degradation path, eventuallyyielding carbon dioxide and water.

MTBE ➔ TBA ➔ Acetone ➔ Formaldehyde ➔ Carbon DioxideC5H12O C4H10O C3H6O CH2O CO2

CASE STUDIES

This article presents two detailed case studies and a summary of three other sites whereozone was used to effectively remove MTBE.

Case Study 1: Ozone Sparge Remediation-Former Service Station

The subject site is an out-of-service, former retail petroleum service station located insoutheastern Pennsylvania. Environmental assessment activities at the site, followingunderground storage tank removal activities, revealed the presence of gasoline floatingon the water table beneath the site and high concentrations of petroleum hydrocarbonsin soil and groundwater.Target compounds exceeding the Pennsylvania Department ofEnvironmental Protection (PADEP) cleanup standards included benzene, toluene,ethylbenzene, xylenes, naphthalene, and MTBE.


© 2002 Wiley Periodicals, Inc. 79

When ozone is dissolvedin groundwater, the

groundwater itselfbecomes a remediating

agent, which can destroycontaminants.

Objective

The subject property was undergoing a real estate transaction as part of the sitedivestiture. As such, the client required that the site be remediated within a nine-monthperiod to facilitate sale of the property.The remedial goal for the site was to removefloating product from the subsurface and reduce adsorbed and dissolved phasehydrocarbons to levels that would allow natural attenuation of remaining contaminantsto achieve site cleanup.

Approach

To ensure that the aggressive timeline would be met for the remediation project, ozonewas selected as the primary remedial technology to be employed at the site. Ozonesparging was conducted through a series of shallow and deep sparge points to destroyhydrocarbon compounds in groundwater and soils.The ozone sparge system wasaugmented with soil vapor and groundwater extraction using total phase extractiontechnology and traditional groundwater pumping.

The remedial approach employed a high-density network of remedial wells, whichfirst involved the application of total phase extraction (TPE) to remove the floatinggasoline product in approximately one month. Soil and groundwater were thenaddressed via dual zone air and ozone sparging, with soil vapor extraction. Afterapproximately four months of these activities, the remedial system was shut off.

Results

The remedial effort at this property vastly exceeded the design goals.The objective forthe remediation system was to remove floating product, reduce soil concentrations to anacceptable risk level, and to reduce dissolved phase concentrations to the low parts permillion (ppm) range to allow natural attenuation to further remediate the site. Oneweek prior to turning on the remedial system, slightly more than 100 gallons of freeproduct was present in an area of approximately 600 square feet. In addition, dissolvedhydrocarbons were present above regulatory standards in an area encompassing over11,000 square feet with BTEX as high as 58,000 ppb and dissolved MTBE as high as17,000 ppb.

The site monitoring wells were sampled one month after shutdown. Only one ofthe 17 wells slightly exceeded the state standards, showing results that substantiallyexceeded the remedial goals.These site wells were sampled again two months afterremedial system shutdown and all were found to comply with State Standards, with adecreasing trend evident between the two rounds.

Three months after system shutdown, sampling of soil and groundwater from tentemporary wells installed between the permanent monitoring points indicated furtherreduction in hydrocarbon compounds. During the installation of these wells, the testborings indicated that the previously dark brown to black clay layers were oxidized to awhitish-gray color, indicative of the reaction of ozone with the organics in the claymaterials and documenting an excellent distribution of the sparge gas.

Finally, six months after system shutdown, groundwater monitoring of source areawells indicated a continued decreasing trend. For this final monitoring event, laboratoryanalyses were expanded from unleaded gasoline to incorporate all priority pollutant



Ozone sparging wasconducted through aseries of shallow and deepsparge points to destroyhydrocarbon compoundsin groundwater and soils.

volatile organic and base neutral compounds.This analysis indicated that no adversecompounds, such as TBA, acetone, or formaldehyde were generated as by-products ofthe ozone application. Statewide health standards for both soils and groundwater wereachieved with dissolved hydrocarbons concentrations continuing to decrease.

The remedial approach removed approximately 3,500 pounds of hydrocarbons fromthe subsurface via active remediation while stimulating subsequent bioremediation throughincreased dissolved oxygen concentrations. BTEX and MTBE concentrations were reducedby 99.75 percent and 98.21 percent, respectively. Exhibit 1 presents these results.

Based on post-remedial site monitoring and sampling, the PADEP issued a NoFurther Action letter with an Act 2 of 1995:The Land Recycling and EnvironmentalRemediation Standards Act (Act 2) Release of Liability.The entire project duration,from remediation system design to No Further Action, was slightly more than one year.

Case Study 2: Ozone Sparge Remediation-Active Service Plaza

The subject site is an active retail petroleum service station located adjacent to a majorhighway in southern New Jersey.The facility is located in a sensitive area of the NewJersey Pine Barrens where the Groundwater Quality Criteria for Class I-PL aquifers isapplicable (Ground Water Quality Standards–New Jersey Administrative Code 7:9-6).Because of the groundwater criteria for the facility there was a high priority toimplement a remediation system that would contain the contaminant plume.

Although BTEX concentrations in groundwater were relatively low to moderate(BTEX at 5,600, 19,000, 2,100, and 16,000, respectively), MTBE was detected atgreater than 400,000 µg/l during the remedial investigation of this site. Tertiarybutyl alcohol (TBA) is also a compound of regulatory concern in New Jersey. TBAwas detected at concentrations as high as 180,000 µg/l. The concentration of MTBEdecreased to about 100,000 µg/l in the source area monitoring well prior toinitiating remediation with ozone. This reduction in MTBE was attributed toattenuation of the plume.



Exhibit 1. Case study 1: hydrocarbon reductions in groundwater.

An ozone oxidation remediation system was designed and installed that combinedthe use of air and ozone in a sparging mode into the groundwater with a soil vaporextraction system for unsaturated zone treatment and off gas control.

Objective

The objective of this remedial application was to reduce dissolved petroleumhydrocarbons throughout the source area and document that the remedial objective wasattained. Reduction of the concentrations of gasoline components within the source areaof the plume would eliminate the flux of components into the down gradient dissolvedgroundwater plume.The reduction of mass flux from the source area, the naturalattenuation, and the concurrent treatment of the dissolved plume would achievecontainment of the plume.These tasks were to be performed in a manner that wouldnot interfere with day-to-day operations of the facility, which is in a high-traffic location.

Remedial Approach

In situ chemical oxidation using ozone was selected as the primary remedial technologycombined with soil venting in the source area of MW-1.The treatment area aroundMW-1 is located directly between the existing tank field and the pump islands.Thesystem was comprised of a total of eight sparge points, six vent wells, and four pointsfor monitoring pressure and vacuum across the treatment area.The remediated area wasapproximately 60 feet × 100 feet.The ozone sparging system was augmented with soilvapor extraction. Recovered VOCs were treated above grade with biofiltration andpolished with conventional vapor phase granular activated carbon.

Results

Sparged ozone clearly affected the oxidation of the contaminants in the groundwater.As expected, the injection of ozone into the groundwater caused an increase indissolved oxygen levels in the groundwater. BTEX and MTBE concentration weresignificantly reduced to extremely low levels over the course of the remediation.During the year-long project, more than 3,500 pounds of contaminants were treatedeither above grade through soil venting or in the subsurface as a result of increasedoxygen levels (in situ chemical oxidation or in situ bioremediation). In MW-1 BTEX,constituents were reduced to non-detectable concentrations, and MTBE and TBA werereduced by over 95 percent to 4,400 and 1,900 µg/l, respectively.The concentrationreductions are shown in Exhibit 2.

Further reductions of MTBE and TBA in MW-1 were not achieved by ozoneoxidation, as there is now a second area to be remediated as a result of an additionalrelease at the site. A portion of the original area that was remediated has beenrecontaminated by this subsequent release and will be addressed as part of a modifiedremedial action plan.

Exhibit 2 presents the reduction of MTBE to TBA as a step in the oxidation process.It appears that during the oxidation of MTBE, concentrations of TBA increased. Aftersufficient MTBE mass remediation,TBA concentrations dropped rapidly as a result ofoxidation of TBA. Subsequent degradation constituents were not observed ingroundwater samples collected from the site. It is theorized that upon formation they



BTEX and MTBEconcentration weresignificantly reduced toextremely low levels overthe course of theremediation.

degraded rapidly to the eventual end products of carbon dioxide and water. It is alsoobserved that upon startup of the ozone oxidation system, concentrations ofconstituents typically increase in groundwater due to desorption and mixing within thesmear and vadose zones.

CASE STUDY SUMMARY

Exhibit 3 presents a summary of projects where ozone was used to remediate effectivelyMTBE under different site conditions.The exhibit demonstrates that the use of ozone toremediate MTBE is repeatable and effective.The sites represented have varyinglithologies and are located across three states.The results demonstrated the ability toachieve remedial objectives with similar durations and ozone application rates.

CONCLUSION

The use of in situ ozone oxidation has proven to be an economical and efficient meansof rapidly cleaning up groundwater contaminated with MTBE and petroleumhydrocarbon contamination. Specifically, results include:

• During these projects the focus and funding was directed towardsremediation and not research. Therefore, additional projects should beperformed where controls such as air and oxygen sparging can be used toevaluate and compare ozone sparging to quantify the MTBE reductionsattributable solely to oxidation.

• The use of ozone sparging at remediation sites has resulted in the cleanup ofsites impacted with light non-aqueous phase liquid (LNAPL) (free product)petroleum products to below regulatory cleanup standards within a period ofmonths.

• TBA is a degradation product of MTBE oxidation, which further degrades as theresult of oxidation.

• Concentrations of constituents in groundwater will initially increase due to



Exhibit 2. Case study 2: hydrocarbon reductions in groundwater.

desorption from soil-adsorbed hydrocarbons and mixing between the vadose andsaturated zone.

• Significant and rapid reductions in contaminant concentrations are not onlyrealized with the volatile petroleum hydrocarbons (benzene, toluene,ethylbenzene, xylenes and cumene), but also with other gasoline constituentsincluding naphthalenes and MTBE.

• Ozone rapidly oxidizes petroleum contaminants to carbon dioxide (CO2) andwater (H2O), leaving no harmful residual by-products.

RELEVANT DOCUMENTS

Squillace, P. J., Zogorski, J. S., Wilber, W. G., & Price, C. V. (1996). Preliminary assessment of the

occurrence and possible sources of MTBE in groundwater in the United States, 1993–1994.

Environmental Science & Technology, 30(5), 1721–1730.

Chang, P., & Young, T. Reactivity and by-products of methyl tertiary butyl ether resulting from water

treatment processes, University of California, Davis [Online]. Available:

http://tsrtp.ucdavis.edu/MTBErpt/vol5_5.pdf. Retrieved June 2002.

Bull, R. A., & Zeff, J. D. (1992). Hydrogen peroxide in advanced oxidation processes for treatment of

industrial process and contaminated groundwater: Chemical oxidation, technologies for the nineties

(pp. 26–36), Lancaster, PA: Technomic Publishing Company.

U.S. Environmental Protection Agency (USEPA) Office of Underground Storage Tanks. (1998). EPA 510-F-

97–015, MTBE fact sheet # 2: Remediation of MTBE contaminated soil and groundwater [Online].

Available: www.epa.gov/OUST/MTBE/. Retrieved June 2002.



Exhibit 3. Summary of projects using ozone to remediate MTBE.

Jeffrey C. Dey, PG, is president and CEO of the Resource Control Corporation (RCC). He works with RCC

teams to improve the level of service and productivity within RCC to meet a diverse set of customer

requirements. Mr. Dey has worked with the RCC team to bring innovative and effective remedial solutions to

the marketplace.

Paul Rosenwinkel, PE, BS Chemical Engineering (University of Illinois, '87), is a vice president of RCC has

contributed scientific and business development skills to Resource Control Corporation for the past decade.

His work focus has been in the areas of on site, in situ remediation innovative technology development and

application, as well as business and organizational development at the growing and vibrant RCC. By

bringing innovation and efficiency to everyday applications of site remediation, he has contributed to the

development and maturing of the environmental site remediation industry.

Kevin Wheeler is an associate vice president and hydrogeology group manager at RCC. He has more than

ten years’ experience in environmental consulting and remediation. The last four and a half years have been

with RCC, where he is responsible for pilot testing, subsurface remedial design and implementation, as well as

remediation system optimization. Prior to RCC, Mr. Wheeler worked for Groundwater Technology, Inc., in both

the Chadds Ford, PA, and Trenton, NJ, offices. He obtained a BS in Geology from Temple University.



in situ remediation of mtbe utilizing ozone

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