S U S A N E . P O W E R S ,

D A V I D R I C E ,

B R E N D A N D O O H E R ,

A N D P E D R O J . J . A LV A R E Z Methyl tert-butyl ether (MTBE) contami-nation in groundwater and surfacewater in the United States is widespreadand currently considered a major threatto drinking water resources (1 3). The

fuel additive is therefore being phased out, and it ishighly likely that ethanol will replace it. To ensurepublic safety, possible impacts to groundwater qual-ity resulting from any releases of ethanol-blendedgasoline must be understood. Typical scenarios in-clude leaks of ethanol-blended gasoline from un-derground storage tanks, accidents with railroad carsor tanker trucks, and spills of neat ethanol at distri-bution terminals.

The potential impacts of such releases were thefocus of an extensive literature review prepared forthe state of California (4) in response to GovernorDavis’s Executive Order calling for the removal ofMTBE from California’s gasoline by December 31,2 0 0 2

Ethanol-BlendedGasoline

Will

AffectGroundwaterQuality?

Using ethanol instead

of MTBE as a gasoline

oxygenate could be

less harmful to the

environment.

VOL. 29, NO. 1, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY � 3 A

(www.calepa.ca.gov/programs/MTBE/EPCResolution.htm). The study indicates critical environmentalfate and transport issues surrounding the use ofethanol-blended gasoline mixtures and serves as abasis for recommending additional research on theenvironmental fate, which would be necessary forthe design of remediation measures to respond toethanol-blended gasoline spills.

MTBE was initially added to gasoline to increaseoctane ratings following the U.S. ban on alkyl leadadditives in 1979. Oil refiners decided to comply withClean Air Act Amendments (CAAA) by using MTBEto formulate oxygenated gasoline (“oxyfuel”) and re-formulated gasoline (RFG) mixtures that contain atleast 2.7% and 2% oxygen by weight, respectively (5).

MTBE’s potential for groundwater and surfacewater contamination was not a significant consid-eration when the decision was made to adopt it asa gasoline oxygenate (5). It is now known, however,that the chemical is a very mobile and persistentcontaminant in aqueous systems, because of its highsolubility and low biodegradation rates. Consumercomplaints of MTBE’s pungent turpentine-like odorand taste effectively limit acceptable levels in pub-lic drinking water supplies to m 5 40 ppb, althoughthere is still significant debate regarding safe levelsof MTBE in drinking water.

In its final report, EPA’s Blue Ribbon Panel on Oxy-genates in Gasoline recommended that the use ofMTBE or any other oxygenate in the nation’s gaso-line be substantially reduced (6). Seven states haveimplemented policies to phase out the use of MTBE,and three states limit the concentration of MTBEthat can be incorporated into gasoline(www.ethanolrfa.org). The federal government hasalso proposed a reduction or a ban on the use of

4 24A � ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 1, 2001

MTBE through provisions in the Toxic SubstancesControl Act (7). To allow for a federal ban on MTBE,EPA has proposed that volatile emissions standardsfrom gasoline be relaxed. This would allow gasolinedistributors more flexibility in their use of ethanol asan alternative.

An obvious replacement?Ethanol is already widely used in oxygenated gaso-line. It is used at concentrations of approximately 68% (by volume) to comply with the CAAA and fre-quently at 10% because of the $0.54/gallon federalexcise tax exemption to promote markets for ethanol(5). Ethanol-blended gasoline is available in manyMidwestern states and currently accounts for morethan 95% of the gasoline sold in the Chicago area(www.ethanolrfa.org/ethanolreport.html). In Brazil,most gasoline contains ethanol.

In separate federal initiatives, ethanol’s use as afuel source is advocated to promote renewable bio-mass fuels. President Clinton signed an ExecutiveOrder in 2000 to accelerate the development and useof biomass fuels, products, and chemicals. The orderincludes funding for both fiscal years 2001 and 2002to encourage increased production at affordableprices. Clinton was also joined by the heads of theAgriculture and Energy Departments, EPA, and Sen-ator Richard Lugar (R-IN) in announcing a goal oftripling bioenergy and bioproducts use by 2010.

Significant changes in U.S. infrastructure wouldbe required for large-scale implementation ofethanol as a gasoline additive whether as an oxy-genate or a biomass fuel. The hygroscopic nature ofethanol-blended gasoline prevents its preparationat a refinery and distribution by pipeline (see side-bar (8)). Thus, distribution terminals receive gasolineand ethanol separately to be mixed as it is pumpedinto a tanker truck for delivery to gasoline stations.

Groundwater quality impactsThere are similarities in the partitioning character-istics of ethanol and MTBE in gasoline (9). The po-larity arising from the oxygen atom in ethanol caus-es it to preferentially partition into water. A similareffect is seen in the polar MTBE molecule, althoughthe solubility of MTBE in water (~4% mass basis) sig-nificantly less than the infinite solubility of ethanol.Like MTBE, ethanol also has a very low tendency toadsorb to mineral surfaces or volatilize from an aque-ous phase. The primary difference between ethanoland MTBE is the former’s high degree of biodegrad-ability. The tert-methyl and ether groups on theMTBE molecule make it difficult to degrade in thesubsurface, whereas ethanol is readily degradedunder both aerobic and anaerobic conditions andlikely will be effectively attenuated under naturalconditions.

Although ethanol is unlikely to be a significantgroundwater pollutant, there is concern regardingthe effects of ethanol on concentrations of othergasoline constituents. Two issues are of most con-cern. Ethanol in water can create a cosolvent effect,and ethanol biodegradation can deplete the ground-water of nutrients and electron acceptors. Both

BTX and ethanol concentrations tracklogarithmically

FIGURE 1

Concentrations of BTX in an aqueous phase equilibrated with gasolineshow a substantial increase in the effective solubility of these constituentswith increasing concentrations of ethanol.

Source: Reprinted from Journal of Contaminant Hydrology, 34, S. E. Heerman, p. 377, 1998, with permissionfrom Elsevier Science.

Aque

ous

phas

eco

ncen

tratio

n(m

g/L)

0.0 0.2 0.4 0.6 0.8

105

104

103

102

10

Aqueous phase volume fraction of ethanol

BenzeneTolueneXylenes

VOL. 35, NO. 1, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY � 25 A

processes could result in an increase of the concen-tration of hydrophobic compounds, such as ben-zene, and thereby increase in the distance that thesehazardous compounds travel from a spill site beforeattenuating processes safely reduce their concen-trations.

Cosolvency issuesAlthough compounds in standard-formulation gaso-line are relatively immiscible in water, ethanol is com-pletely miscible in both gasoline and water at all con-centrations. With a sufficiently large amount ofethanol in a localized subsurface environment, gaso-line and water become completely miscible with eachother and merge into a single phase (see sidebar). Al-though this might occur following a spill of neatethanol into petroleum-contaminated soil at a ter-minal, ethanol concentrations (m15% by volume inthe aqueous phase) in groundwater near the site ofan RFG spill are expected to be much lower than the80% by volume concentrations in the aqueous phasethat is required to create a single phase (10, 11).

Aqueous-phase concentrations of ethanol thatleach from an RFG or oxyfuel spill could also be highenough to increase the groundwater concentrationsof individual chemical species equilibrated with thegasoline. Of most concern are the monoaromatic hy-drocarbons, especially the carcinogen benzene.Ethanol also affects equilibrium partitioning rela-tionships by the “cosolvent effect”, which is observedas a reduction of the aqueous-phase polarity whenhigh concentrations of polar organic compounds are

present.Heermann and Powers (10) studied the cosolu-

bility effects associated with ethanol in gasoline. Re-sults of batch equilibrium experiments illustrate anapproximate logarithmic increase in benzene,toluene, ethylbenzene, and xylene (BTEX) concen-trations with increasing ethanol concentrations (seeFigure 1). Over the range of the maximum aqueous-phase ethanol volume fractions observed by Corseuiland Fernandes (11) (Û15%), benzene, toluene, andxylene concentrations generally increase by approx-imately 20 60%. The smallest percentage increasewas observed for benzene, the least hydrophobic ofthe BTEX compounds. More hydrophobic com-pounds in gasoline, such as xylene will have a greaterpercentage increase in their aqueous-phase con-centrations in the presence of ethanol. These resultssuggest that it is unlikely that cosolvent-related in-creases in BTEX concentrations will be significantrelative to other processes that field-scale concen-trations following a spill of ethanol-blended gaso-line. Spills of neat ethanol at a bulk terminal, how-ever, could result in very high ethanol concentra-tions in a localized area and a much more significantpossibly an order of magnitude increase in BTEXconcentrations.

Similar to the cosolvent effects described above,the presence of ethanol in groundwater reduces theextent of sorption of hydrophobic compounds be-cause of the reduced polarity of the aqueous phase.Although this phenomenon has not been studied forethanol in gasoline, studies with other cosolvents in-

The miscibility of ethanol gasoline watermixturesEthanol-blended gasoline requires unique handlingbecause it tends to separate into two phases whencontaminated by water. Ethanol partitioning and theeffects of ethanol on solubility can be illustrated on aternary phase diagram. The shaded region (see figurebelow) indicates the range of water, gasoline, andethanol mass fractions where the three componentsexist as two separate phases. The unshaded regionindicates the composition where these componentsexist as a single phase. For example, if a gasoline ini-tially contained 10% (by mass) ethanol, its compositionwould be indicated by point A. If additional waterenters the gasoline storage tank or automobile gastank, the overall composition would change. Severalthousand parts per million of water can dissolve intoethanol-blended gasoline. However, if this solubilitylimit is exceeded and the overall mixture now has massfractions that are within the shaded area, the gasolinewill separate into two phases. A gasoline-rich phase(B) will float on top of a water and ethanol mixture (B ).A significant drop in temperature can also inducephase separation in ethanol-blended gasoline becauseof the reduced solubility of water and ethanol in gaso-line at lower temperatures. This is why separate bulkstorage tanks at terminals are needed, and pumpingethanol by pipeline is difficult.

Ternary phase diagram for gasoline–ethanol–watersystem at 21 ∞CThe shaded portion of the diagram represents the region where thetotal mass fractions separate into two phases. The ends of the dashed(tie) lines indicate the composition of each phase at equilibrium. Axesrepresent mass percentages.

Source: Adapted from (8) with permission.

100%ethanol

Wat

er

Ethanol

100%gasoline

100%water Gasoline

0 20 40 60 80 100100

B’80

60

40

20

0 100

80

60

40

20

0B

A

graded by indigenous microbes. Table 1 presents thehalf-life of ethanol for a variety of redox conditionsas determined from laboratory microcosm studies.Conditions in an aquifer (lower microbial concen-trations, colder temperatures, depletion of electronacceptors, etc.), would likely result in longer sub-surface half-lives in situ than those depicted inTable1. Nevertheless, it is expected that ethanol will biode-grade much faster in the subsurface than other gaso-line constituents.

Because of its high oxygen demand, ethanol islikely to be degraded predominantly under anaero-bic conditions. None of the products of anaerobicethanol degradation is toxic, although some metabo-lites (e.g., butyrate, methane) and changes in inor-ganic chemical speciation caused by the alteredredox potential could have adverse aesthetic impacts.In addition, acetate and other volatile fatty acidscould lower pH and inhibit microbial activity if theyaccumulate at high concentrations in poorly bufferedsystems. Such effects are likely to be system-specif-ic due to variability in buffering and dilution capac-ity among contaminated sites.

Biodegradation is often the single most impor-tant mechanism attenuating the subsurface migra-tion of gasoline constituents, such as BTEX (17).Therefore, quantifying BTEX biodegradation rates isnecessary to predict the net transport of these pol-lutants from a gasoline spill and to evaluate the po-tential risks to groundwater resources.

BTEX biodegradation, however, is integrally linkedto the biodegradation of ethanol. Ethanol is likely tobe preferentially degraded by the indigenous mi-croorganisms that would otherwise feed on BTEX(14, 16). Moreover, with ethanol concentrations ex-pected in the thousands of parts per million, this or-ganic solute could exert a greater biochemical oxy-gen demand than other soluble components of gaso-line. Thus, ethanol is likely to accelerate the deple-tion of dissolved oxygen and decrease the extent ofaerobic BTEX degradation. This is particularly im-portant for the fate of benzene, which is the mosttoxic of the BTEX constituents (18). When degradedin situ by indigenous microorganisms, benzene canhave a half-life that can vary from an ~2 days in anaerobic aquifer to more than 200 days under anaer-obic conditions (19). Depending on aquifer chem-istry and the rate of natural replenishment of elec-tron acceptors, ethanol could also deplete the elec-tron acceptors and nutrients needed for fully de-grading ethanol and BTEX species.

Overall fate of mixturesField studies, in conjunction with comprehensivemodeling, are needed for evaluating the net impactof cosolvency and biodegradation effects on ground-water quality. Researchers have conducted somemodeling studies, but always with significant as-sumptions about the significance of cosolvency orbiodegradation mechanisms (4, 15, 20). For example,many models assume that BTEX biodegradationdoes not occur in areas where ethanol is present atconcentrations above some threshold value.

Nevertheless, the conclusions drawn from various

26A � ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 1, 2001

dicate that the cosolvency effect is less significantfor sorption than for solubilization (12, 13). Thus, itappears that ethanol would have little impact on theretardation of BTEX species in an aquifer, at least atthe low ethanol concentrations expected for ethanol-blended gasoline spills. The impact of ethanol onmore hydrophobic gasoline components is notknown at this time.

Biodegradation issuesEthanol can be degraded at a faster rate in both aer-obic and anaerobic environments than other gaso-line constituents (14, 15). Large concentrations(m100,000 mg/L) of ethanol are not biodegradableand are rather toxic to most microorganisms (16).Such high concentrations are unlikely to be en-countered at sites contaminated with ethanol-blend-ed gasoline. Thus, ethanol plumes should be de-

Predicted contaminant plume length

FIGURE 2

Results of Monte Carlo simulations illustrate relative changes in theprobability that a drinking water well will be affected following a spill ofnonoxygenated gasoline (base case), ethanol-blended gasoline, and MTBE-blended reformulated gasoline. Analysis details are presented in (4).

Source: Adapted from reference (4).

Rela

tive

chan

gein

prob

abili

tyth

ata

wel

lw

illbe

impa

cted

0 20 40 60

100%

Time (years)

50%

0%

MTBE,95th percentile

Benzene with ethanol,95th percentile

Benzene, 95th percentile—Base case

VOL. 35,, NO. 1, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY � 27 A

modeling studies suggest that benzene will indeedtravel farthest from the site of an ethanol-blendedgasoline spill. Predictions generally show that thesebenzene plumes range from 20 to 150% longer com-pared with nonoxygenated gasoline. Variability inpredicted plume lengths depends on the modelingstudy assumptions and the nature of the aquiferand transport properties investigated.

AnalysisMonte Carlo simulations were used to predict a dis-tribution of contaminant plume lengths for threerisk scenarios: benzene in groundwater from a stan-dard gasoline spill; benzene in groundwater from anethanol-blended gasoline spill (5.7% by volumeethanol in the gasoline); and MTBE in groundwaterfrom an MTBE-blended RFG spill. The plume lengthdistributions were then used to estimate the proba-bility that a drinking water well near a spill site wouldbe impacted. Although many simplifying and con-servative assumptions were made in this analysis in-cluding that all releases occur simultaneously theresults provide a qualitative comparison of an un-derstanding of the risks of well contamination dueto the use of oxygenated gasolines. Figure 2 showsthat, relative to risks associated with standard for-mulation gasoline, there is an increase in the riskthat wells will be contaminated by RFG using eitherMTBE or ethanol as an oxygenate.With ethanol, therisk of contaminating wells decreases after approx-imately five years. However, the risk continues togrow for MTBE because of the assumption that thischemical is not degraded in the subsurface. The con-servative approach used in this analysis, includingthe low biodegradation rates and assumption thatthe gasoline source areas are not remediated, resultsin an overstatement of the risks associated with theseadditives to gasoline. Nevertheless, the relative trendsdo favor ethanol when considering risks associatedwith RFG spills.

More knowledge neededThe California-sponsored study (www.calepa.ca.gov/programs/MTBE/EPCResolution.htm) presentsextensive knowledge about the fate and impacts ofethanol-blended gasoline in the subsurface, butmany physical, chemical, and biological processes af-fecting concentrations of gasoline constituents arestill not adequately understood. Although modelingstudies (4, 15, 20) show that assumed biodegradationrates have a significant impact on the predicted in-crease in benzene plume lengths, currently availableresearch results quantifying these rates are limitedin number and scope. Additional studies are clearlyneeded. Ongoing work at Lawrence Livermore Na-tional Laboratory and the University of Iowa shouldimprove our ability to understand and predict howsuch substrate interactions affect degradation ki-netics. Other areas needing research are the rates ofethanol partitioning from a gasoline spill to ground-water and the nature of gasoline pools when ethanolis present or spilled into an existing pool. Experi-mental, modeling, and field studies would all be ap-propriate for increasing our understanding of these

processes.The lack of a historical database confounds ef-

forts to understand and predict the impacts ofethanol on groundwater quality. A telephone survey(4) of leaking underground fuel tank regulators inMidwestern states revealed that they know they havehad spills of gasohol, but these cannot be tracked,because databases archiving spill histories general-ly do not differentiate among gasoline types. Thelack of any regulatory requirement to test for ethanolalso contributes to data scarcity.

There are lessons to be learned from MTBE use.The ubiquity of MTBE in the environment was notunderstood until we started looking for it. Analyti-cal techniques for assessing ethanol concentrationsare now available. This analyte, as well as other an-alytes that indicate theredox state of the aquifershould be routinely moni-tored at gasoline-impactedsites, especially in the Mid-west and California, whereethanol is already in use. Asthe federal government andstate governments considerlegislation to increase theuse of ethanol, the potentialenvironmental impacts ofvarious possible fuel oxy-genates weighed against anticipated benefits shouldbe a component of these policy decisions.

EPA’s Blue Ribbon Panel (6) indicated that life-cycle analysis must be a key element of quantitativeassessments used by policy makers for risk and costbenefit analyses comparing alternative motor vehi-cle fuels. However, no organization is likely to haveresources sufficient to address all the research andassessment needs required to perform these com-parisons before new policies are implemented. Still,given the choice between MTBE and ethanol, andwith our current level of understanding, it appearsthat the groundwater resource impacts associatedwith the use of ethanol will be less significant andmore manageable than those associated with MTBE.

AcknowledgmentsThis article was prepared from the review complet-ed for the California Environmental Policy Councilby Lawrence Livermore National Laboratory underthe auspices of the U.S. Department of Energy (Con-tractW-7405-Eng-48). Much of the work cited in thisreport was conducted with additional financial sup-port from the U.S. EPA Science to Achieve Resultsprogram in the National Center for EnvironmentalResearch and Quality Assurance (Grant numbersR821114 and R82815601) and the American Petrole-um Institute.

References(1) Zogorski, J. S. Fuel Oxygenates and Water Quality Find-

ings and Recommendations of the Interagency Oxy-genated Fuel Assessment. In Proceedings of the Petro-leum Hydrocarbons and Organic Chemicals in GroundWater Prevention, Detection, and Remediation confer-ence, Houston, TX, Nov. 12 15, 1997; National GroundWater Assocaition: Dublin, OH, 1997; pp. 3 8.

(2) Gullick, R. W.; LeChevallier, M. W. J. Am. Water Works

Pull quote?

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Remediation, Houston, TX, Nov. 1994, National GroundWater Association: Dublin, OH, 1994; pp. 75 90.

(14) Corseuil, H. X., et al. Water Res. 1998, 32 (7), 2065.(15) Malcolm Pirnie, Inc. Evaluation of the fate and transport

of ethanol in the environment; Report prepared for theAmerican Methanol Institute, http://www.methanol.org/reformgas/index.html (accessed Nov. 2000).

(16) Hunt, C. S.; Alvarez, P. J. J.; dos Santos Ferreira, R.; Corseuil,H. X. Effect of Ethanol on Aerobic BTEX Degradation. InProceedings of the Fourth Internatioanl In Situ and On-Site Bioremediation Symposium, Vol. 4, No. 1, New Or-leans, LA, Apr. 1997; Batelle Press: Columbus, OH: pp. 4954.

(17) National Research Council. Natural Attenuation forGroundwater Remediation; National Academy Press:Washington, DC, 1999.

(18) Alvarez, P. J. J.; Vogel, T. M. Water Sci. Technol. 1995, 31(1), 15 22.

(19) Suarez, M. P.; Rifai, H. S. Bioremed. J. 1999, 3 (4), 337 362.(20) Molson, J. W.; Barker, J.; Schirmer, M. Modeling the im-

pact of ethanol on the persistence of BTEX compoundsin gasoline-contaminated groundwater; Report preparedfor the National Water Research Institute (NWRI), Re-port No. NWRI-99-08; NWRI: Fountain Valley, CA, 1999.

Susan E. Powers is an associate professor in the De-partment of Civil and Environmental Engineering,Clarkson University, Potsdam, NY 13699-5710. DavidRice is the Environmental Chemistry and Biology Groupleader, and Brendan Dooher is a systems analyst andrisk assessor in the Environmental Protection Depart-ment at Lawrence Livermore National Laboratory, Liv-ermore, CA 94550.Pedro J. J. Alvarez is an associate pro-fessor in the Department of Civil and EnvironmentalEngineering,TheUniversity of Iowa, IowaCity, IA 52242-1527.Youmay contact Susan Powers at (315) 268-6542;fax: (315) 268-7985; or e-mail: sep@clarkson.edu.

Assoc., 2000, 92 (1), 100 113.(3) Johnson, R., et al. Environ. Sci. Technol. 2000, 34 (9),

210A 217A.(4) Rice, D.W. Health and Environmental Assessment of the

Use of Ethanol as a Fuel Oxygenate, Volume IV Poten-tial Ground and Surface Water Impacts; UCRL-AR-135949, http://www-erd.llnl.gov/ethanol (accessed Nov.2000).

(5) Franklin, P. M., et al. Environ. Sci. Technol. 2000, 34 (18),3857 3863.

(6) Blue Ribbon Panel on Oxygenates in Gasoline. Achiev-ing Clean Air and Clean Water: The Report of the BlueRibbon Panel on Oxygenates in Gasoline; EPA 420-R-99-021; U.S. Government Printing Office: Washington, DC,1999.

(7) Fed. Regist. 2000, 65 (58), 16,094 16,109.(8) de Oliviera, E. Ethanol Flushing of Gasoline Residuals

Microscale and Field-Scale Experiments. Ph.D. Disser-tation, University of Waterloo, Waterloo, Ontario, Cana-da, 1997.

(9) Powers, S. E., et al. CRC Critical Reviews in Environ-mental Science and Engineering CRC Press, Boca Raton,FL, in press.

(10) Heermann, S. E.; Powers, S. E. J. Contam. Hydrol. 1998,34 (4), 315 341.

(11) Corseuil, H.X.; Fernandes, M. Cosolvency Effect inAquifers contaminated With Ethanol-Amended Gaso-line, In Natural Attenuation of Chlorinated PetroleumHydrocarbons, and Other Organic Compounds. Pro-ceedings of the Fifth International In Situ and On-siteBioremediation Symposium, San Diego, CA, Apr. 1999,Alleman, B. C., Leeson, A., Eds.; Battelle Press: Colum-bus, OH, 1999; pp. 135 140.

(12) Fu, J. K.; Luthy, R. G. J. Environ. Eng. 1986, 112 (2), 346.(13) Rixey, W. G. The Effect of Oxygenated Fuels on the Mo-

bility of Gasoline Components in Groundwater. In Pro-ceedings of the Petroleum Hydrocarbons and OrganicChemicals in Groundwater: Prevention, Detection, and

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