mtbe and other oxygenates: environmental sources, analysis, occurrence, and … · 2003. 9. 18. ·...

ENVIRONMENTAL ENGINEERING SCIENCEVolume 20, Number 5, 2003© Mary Ann Liebert, Inc.

MTBE and Other Oxygenates: Environmental Sources,Analysis, Occurrence, and Treatment

Rula A. Deeb,1,* Kung-Hui Chu,2 Tom Shih,3 Steven Linder,4 Irwin (Mel) Suffet,5

Michael C. Kavanaugh,1 and Lisa Alvarez-Cohen6

1Malcolm Pirnie, Inc.Emeryville, CA 94608

2Civil and Environmental EngineeringUniversity of Tennessee

Knoxville, TN 37996-20103California EPA

State Water Resources Control BoardLos Angeles, CA 90013

4EPA Region 95Environmental Science and Engineering Program

School of Public HealthUniversity of California

Los Angeles, CA 90095-17726Civil and Environmental Engineering

University of CaliforniaBerkeley, CA 94720-1710

ABSTRACT

The production and use of fuel oxygenates has increased dramatically since the early 1990s due to fed-eral and state regulations aimed to improve air quality. Currently, methyl tert-butyl ether (MTBE) is themost widely used oxygenate in gasoline, followed by ethanol. Widespread use of oxygenates in gasolinehas been accompanied by widespread release of these materials into the environment. This manuscriptprovides a review of environmental sources of MTBE and alternative oxygenates, analytical methods avail-able for their detection in environmental samples, their occurrence in the environment with a focus ongroundwater, and treatment methods for their removal from gasoline-contaminated water. Accidental gaso-line releases from underground storage tanks and pipelines are the most significant point sources of oxy-genates in groundwater. Because of their polar characteristics, oxygenates migrate through aquifers withminimal retardation, raising great concerns nationwide of their potential for reaching drinking watersources. As a group, fuel oxygenates present distinct analytical and sample preparation issues. Conven-tional procedures for the analysis of gasoline constituents have been shown to be insensitive for fuel oxy-

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*Corresponding author: Malcolm Pirnie, Inc., 2000 Powell Street, Suite 1180, Emeryville, CA 94608. Phone: 510-735-3005;Fax: 510-596-8855; E-mail: [email protected]

INTRODUCTION

FUEL OXYGENATES have been used in the United Statesfor more than 2 decades (Squillace et al., 1995). Inthe late 1970s, oxygenates were added to gasoline as oc-tane enhancers following the phase-out of tetra-ethyllead. In the United States, the addition of oxygenates togasoline increased significantly following the Clean AirAct Amendments (CAAA) of 1990, which mandated theuse of reformulated and oxygenated gasoline in certainurban regions to reduce air pollution from motor vehi-cles. To meet the requirements of the CAAA, the UnitedStates Environmental Protection Agency (U.S. EPA) ini-tiated the Oxyfuel Program in 1992 and the ReformulatedGasoline Program (RFG) in 1995. The former requiredthe use of gasoline with 2.7% oxygen by weight duringthe winter months to control carbon monoxide emissions,and the latter required the use of gasoline with 2% oxy-gen throughout the year in ozone nonattainment areas. Inaddition, some cities elected to use oxygenate-blendedfuels to enhance air quality (U.S. EPA, 1998a).

Fuel oxygenates can be divided into two chemical cat-egories: ethers and alcohols. Ether oxygenates approvedby the U.S. EPA include methyl tert-butyl ether (MTBE),ethyl tert-butyl ether (ETBE), tert-amyl methyl ether(TAME), and diisopropyl ether (DIPE). Alcohol oxy-genates include ethanol (EtOH), tert-butyl alcohol(TBA), and methanol (MeOH). The molecular structuresof these compounds are provided in Fig. 1 and key phys-ical and chemical properties are provided in Table 1.

Although the 1990 CAAA required the addition ofoxygenates to gasoline, the use of a specific oxygenatewas not designated. Historically, the most widely usedoxygenate in the United States was MTBE because of itshigh octane level, low cost of feedstocks (MeOH and iso-butylene), low production cost, ease of blending withgasoline, and ease of transfer and distribution. In refor-mulated gasoline, the percentage of MTBE on a volumebasis can be as high as 11 to 15% (U.S. EPA, 1998a). In

the United States alone, the annual consumption ofMTBE exceeds four billion gallons on an annual basis.MTBE is one of the most commonly manufactured chem-icals in the United States and is used almost exclusivelyin gasoline.

EtOH is the second most widely used oxygenate in theUnited States. The EtOH fuel industry was created in1978 in the United States with the passage of the EnergySecurity Act. The purpose of this program was to increasegasoline availability during the oil embargo and to de-crease dependence on imported oil. In 1978, a Nebraskagroup marketed gasohol, a gasoline containing 10%EtOH by volume. By 1992, 8% of the gasoline in theUnited States contained EtOH. In 1998, 15% of all U.S.oxygenate-blended gasoline contained EtOH. Of theEtOH used in gasoline, roughly 30% goes to RFG dur-ing the summer, 20% to Oxyfuel in the winter, and 50%to conventional gasoline year round to enhance octaneand extend fuel supplies (U.S. EPA, 1998a).

Due to recent problems involving the contamination ofdrinking water resources by MTBE in the United States,legislation in several states has called for its removal fromgasoline. The extent to which alternative oxygenates are

434 DEEB ET AL.

genates, and ether hydrolysis to alcohol under acidic conditions has led to a reassessment of conventionalhandling techniques for groundwater samples. An evaluation of MTBE’s occurrence in drinking watersources over time in three states showed that the frequency of MTBE detection since 1999 appears to bestabilizing in groundwater and slightly decreasing over time in surface water. Recent studies have dem-onstrated the effectiveness of conventional treatment technologies and the promise of emerging technolo-gies for MTBE removal from contaminated media. However, the removal from water of tert-butyl alco-hol (TBA), an impurity in MTBE-blended fuels and an MTBE breakdown product, can be problematicusing some conventional technologies such as air stripping and granular activated carbon. These limita-tions may generate additional problems for water purveyors, regulators, and site managers.

Key words: MTBE; TBA; ethanol; fuel oxygenates; groundwater; contamination; treatment

Figure 1. Molecular structures of EPA-approved fuel oxy-genates.

used depends on chemical availability, performance char-acteristics, production and distribution costs, and poten-tial environmental impacts. In general, ether oxygenatessuch as DIPE, ETBE, and TAME are water soluble andrelatively persistent in the environment. Therefore, as aclass of compounds, future uses of ethers may be limitedby the same water quality concerns that led to the phase-out of MTBE. The use of MeOH, an alcohol oxygenate,may also be limited by its toxicity relative to other oxy-genates. Finally, the use of TBA may be restricted by itstoxicity and relatively low octane. As a result, EtOH isthe most likely replacement for MTBE, and its use is ex-pected to increase in the United States over the next sev-eral years to comply with CAAA oxygenate require-ments.

In contrast to the United States, fuel oxygenates in Eu-rope are primarily used as octane enhancers. EuropeanMTBE production began in 1973, and increased over timeto current annual levels of approximately 2.2 million tons,with an expected increase to 3.8 million tons over thenext 5 years. However, due to growing concern regard-ing groundwater contamination by MTBE, alternative oc-tane enhancers have been proposed for use by some Eu-ropean countries. Although TAME and ETBE are alreadyin use, the introduction of ethanol is being discussed(Schmidt et al., 2001).

This paper provides an overview of the uses of oxy-genates in gasoline, environmental sources of these com-pounds (point and nonpoint), analytical methods avail-able for their detection, their environmental occurrence,

and technologies available for their removal from con-taminated soil and water. This paper focuses primarilyon MTBE, with only some discussion of other ether oxy-genates and EtOH because there is a limited body of pub-lished information on them.

SOURCES

Like other components of gasoline, oxygenates can beintroduced into the environment during all phases of thepetroleum fuel cycle. Major environmental sources ofoxygenates are associated with the distribution, storage,and use of oxygenate-blended fuels. Releases of gasolineor other fuels containing oxygenates include pointsources such as (1) releases from underground storagetanks and pipelines, (2) spills at industrial plants and re-fueling facilities, and (3) accidental spills during trans-port. Nonpoint sources of oxygenates include exhaust andevaporative emissions from vehicles, stormwater runoffand atmospheric deposition. An evaluation of the relativesignificance of the potential sources of oxygenates fol-lowing their release to the environment is discussed below.

Point sources

Storage tank releases. Underground storage tanks(UST) for oxygenated gasoline likely pose the most se-rious threat to groundwater contamination. The U.S. EPAtracks national leaking underground storage tank (LUST)

MTBE AND OTHER OXYGENATES 435

ENVIRON ENG SCI, VOL. 20, NO. 5, 2003

Table 1. Key physical and chemical properties of fuel oxygenates.

MTBE a ETBE b TAME c DIPE d TBAe Ethanol Methanol

Molecular 88.15 102.18 102.18 102.18 74.12 46.07 32.04Weight(g/mol)

Water 43,000– ,26,000 ,20,000 2,039 Miscible Miscible MiscibleSolubility 54,300 9,000(mg/L) @ 20°C@ 25°C

Vapor 245–256 152 68.3 149–151 40–42 49–56.5 122Pressure @ 20°C(mmHg)@ 25°C

Log Kocf 1.035– 0.95– 1.27– 1.46– 1.57 0.20–1.21 0.44–0.92@ 25°C 1.091 2.20 2.20 1.82

Hcg 0.024– 0.109 5.19 3 0.195– 4.25 3 1024 2.10 3 1024– 1.09 3 1024

(2) 0.122 1022 0.408 5.93 3 1024 2.57 3 1024

@ 25°C

Source: Data adapted from Zogorski et al. (1997). aMTBE: methyl tert-butyl ether; bETBE: ethyl tert-butyl ether; cTAME: tert-amyl methyl ether; dDIPE: diisopropyl ether; eTBA:

tert-butyl alcohol; fKoc: organic carbon partitioning coefficient; gHc: Henry’s Law constant.

information in the Corrective Action Database, whichcontains quarterly (1993–1995) or semiannual (1996–present) monitoring information (www.epa.gov/swerust1/cat/camarchv.htm). The State of California compiles USTdata in the Leaking Underground Storage Tank Informa-tion System, which is available online and was recentlyupdated to contain MTBE information (www.swrcb.ca.gov/cwphome/lustis). For example, in 1998, the cu-mulative number of identified LUSTs totaled 37,550 withan estimated 22,790 containing MTBE. Approximately2,420 of these tanks contaminated surrounding ground-water with MTBE. In addition, 790 nearby surface wa-ter sources tested positive for MTBE.

Although statistics on relative quantities of MTBE re-leased from all sources are lacking, the relative numberand magnitude of releases from LUSTs clearly suggeststhat LUSTs are the most significant source of MTBE insubsurface environments.

Industry releases. Estimated releases of MTBE fromindustrial activities are reported to the U.S. EPA in theannual Toxics Release Inventory (www.epa.gov/tri). In-dustrial MTBE releases according to the TRI are pre-sented in Table 2. Refineries account for most of the in-dustrial MTBE releases, and most of the reported releasesare to the atmosphere. In addition, the majority of the re-leases occurred on-site. Interestingly, there is no distinctcorrelation between the mass of MTBE released and theincrease in MTBE usage after the Clean Air Act Amend-ments of 1992.

Other. Other point sources of MTBE in the environ-ment include releases from recreational vehicles, and ac-

cidental spills during transport and fueling. Surface wa-ter sources are impacted by MTBE primarily as a resultof recreational boating. Releases of unburned fuel fromwatercrafts during recreational activity are most prob-lematic for inefficient two-stroke engines, which can re-lease up to 30% of their fuel unburned (Reuter et al.,1998). However, modeling studies suggest that MTBE isnot likely to persist in surface water bodies because ofvolatilization (Pankow et al., 1996; Stocking and Ka-vanaugh, 2000).

Due to limited or unavailable nationwide data, it isnot possible to quantify MTBE releases during trans-port, for example, tank ships and barges, trucks, traincars, facilities, and pipelines. For example, although theU.S. Coast Guard tabulates spill data from 1969 to thepresent for a range of transportation incidents related tothe transport of hazardous and nonhazardous chemicalsand oil, the data are reported in broad categories (suchas liquid chemical spills and oil spills) rather than forspecific chemicals (www.uscg.mil/hq/g-m/nmc/response/stats/ac.htm).

Small spills, which are defined as less than 1 gallon,may occur at various stages of fuel handling. Numerousdiscreet spills associated with refueling have been sug-gested as a significant mechanism for groundwater con-tamination (Zogorski et al., 1997; Maine Department ofHealth Service, 1998). For example, MTBE was detectedin groundwater in Maine in areas that did not containLUSTs. However, a recent modeling analysis (Flores etal., 2000) suggested that small refueling spills do not rep-resent a significant threat to groundwater because the ma-jority of MTBE would be expected to volatilize prior toreaching the groundwater table. In contrast, discrete spills

436 DEEB ET AL.

Table 2. On-site and off-site reported U.S. releases of MTBE in kilograms.

Total air Surface water Underground Releases to Total on-site Total off-site Total on- andYear emissions discharges injection land releases releases off-site releases

1988 1,176,476 9,772 6,545 168 1,192,962 2,092 1,195,0541989 1,465,006 17,018 8,773 586 1,491,384 2,101 1,493,4851990 1,353,139 19,395 51,091 682 1,424,307 3,498 1,427,8051991 1,486,419 14,047 37,132 1,320 1,538,917 2,885 1,541,8021992 1,426,950 46,759 31,111 131 1,504,951 6,968 1,511,9191993 1,718,247 42,846 4,275 186 1,765,554 61,060 1,826,6141994 1,449,347 41,882 13,475 1,012 1,505,715 53,524 1,559,2401995 1,500,345 35,707 6,926 1,727 1,544,705 21,746 1,566,4511996 1,369,909 52,891 80,534 12,077 1,515,410 110,555 1,625,9651997 1,196,632 74,416 7,418 56 1,278,523 53,691 1,332,2141998 1,899,685 30,823 21,640 1,459 1,953,606 119,727 2,073,3331999 1,699,590 55,050 9,308 3,003 1,766,950 117,515 1,884,4652000 1,569,856 56,376 14,198 4,798 1,645,229 14,697 1,659,926

Source: Data downloaded (in pounds) from USEPA’s Toxic Releases Inventory at www.epa.gov/triexplorer/chemical.htm

http://www.epa.gov/swerust1/cat/camarchv.htmhttp://www.epa.gov/swerust1/cat/camarchv.htmhttp://www.swrcb.ca.gov/cwphome/lustishttp://www.swrcb.ca.gov/cwphome/lustishttp://www.epa.gov/trihttp://www.epa.gov/triexplorer/chemical.htmhttp://www.uscg.mil/hq/g-m/nmc/response/stats/ac.htmhttp://www.uscg.mil/hq/g-m/nmc/response/stats/ac.htm

from ruptured gas tanks or backyard disposal are ex-pected to result in detectable concentrations of MTBE ingroundwater. The impact of such releases to groundwa-ter is strongly dependent on site hydrogeology. Ulti-mately, it is estimated that small spills represent a minorsource of MTBE contamination to soil and groundwatercompared to releases from LUSTs (Flores et al., 2000).

Although the use of MTBE in fuels is typically re-stricted to gasoline, recent studies on fuels in Connecti-cut (Robbins et al., 2000a, 2000b) confirmed the pres-ence of MTBE in heating oil (at 9.7 to 906 mg/L) anddiesel fuel (74 to 120 mg/L). Further, a review of 78 casefiles involving heating oil releases reported MTBE ingroundwater at 73% of the sites, with concentrations ex-ceeding the federal drinking water advisory levels of 20mg/L at 32 to 46% of the sites (Robbins et al., 1999). Thesource of MTBE in these heating oils and diesel fuels isnot fully known but is likely to be related to mixing ofsome gasoline with other fuels in incompletely emptiedtanker trucks and other transport facilities. Also, it is notknown whether this type of impurity is a nationwide phe-nomenon.

Nonpoint sources

In addition to point sources, MTBE can contaminatewater resources from nonpoint sources such as atmo-spheric washout and urban runoff. MTBE enters theatmosphere from exhaust and evaporative emissions.Once in the atmosphere, MTBE is primarily removed byphotochemical degradation and to a much lesser extent,by precipitation (Zogorski et al., 1997). In urban settings,the partitioning of MTBE to precipitation can result inlow but detectable (.1 ppb) concentrations of MTBE insurface water. This is suspected to be the case in shallowurban groundwater in areas such as Denver, New En-gland, and elsewhere (Baehr et al., 1997; Pankow et al.,1997). Although a study on urban runoff of MTBE re-ported that 7% of 592 stormwater samples tested posi-tive for MTBE (Grady and Casey, 1998), a subsequentstudy of dry season urban runoff suggested that only asmall percentage of MTBE discharged to receiving wa-ter bodies occurred by this mechanism (Brown et al.,2001).

ANALYTICAL METHODS

Over the past several years, U.S. EPA started requir-ing the monitoring and reporting of oxygenates ingroundwater at all LUST sites nationwide. The selectionof the appropriate analytical method depends on the tar-get oxygenates. Fuel oxygenates are typically dividedinto two groups for analytical purposes, ether oxy-

genates/TBA and EtOH/MeOH. The most commonlyused methods for ether oxygenates and TBA include EPAmethods 8021 (photoionization detection) and 8260(mass spectrometry), and to a lesser extent method 8015(flame ionization detection) (Jaros, 2001; Rhodes andVerstuyft, 2001). However, the less expensive 8021 and8015 methods are not as effective for MTBE, and con-sequently should be verified with EPA method 8260.

A recent study evaluated three purge-and-trap meth-ods commonly employed at LUST sites (Halden et al.,2001). Consistently, good results were achieved withEPA method 8240B/60B (mass spectrometry) and ASTMmethod D4815 (flame ionization detection) for MTBE,TBA, ETBE, TAME, and DIPE in reagent water withgasoline. EPA method 8020A/21B (photoionization de-tection) was reported to be unfit for TBA analysis be-cause it yielded false positives for MTBE when TPH lev-els exceeded 1 mg/L. For all three methods, detectionlimits were sufficiently low (ppb levels). In addition, amethod using purge-and-trap followed by gas chro-matography/mass spectrometry operated in the selectedion monitoring mode yielded a detection limit for MTBEin the low part-per-trillion range (Ekwurzel et al., 2001).

EtOH and MeOH are more difficult to determine inenvironmental samples because these compounds arehighly polar, and therefore difficult to measure usingpurge-and-trap techniques. Although direct aqueous in-jection based gas chromatographic methods (EPA 8260or a combination of methods 5031 and 8260) are effec-tive in measuring part-per-million levels of EtOH andMeOH, the sensitivity is limited by the small injectionvolume and the presence of water, which affects both thecolumn and detector performance (California MTBE Re-search Partnership, 1999; Achten and Puttmann, 2000;Cassada et al., 2000; Black and Fine, 2001). Analyticalmethods involving solid phase microextraction (SPME)are promising for EtOH and MeOH, as well as for etheroxygenates. SPME involves the partitioning of analytesfrom water to a sorbent material bound to a fused silicafiber, which is then placed into a narrow-bore gas chro-matograph for thermal desorption and analysis. The se-lectivity of SPME for different classes of compounds inhighly dependent on the nature of the sorbent utilized,but detection limits at the part-per-billion levels and be-low were achievable (Black and Fine, 2001).

In an effort to provide technical information regardingappropriate sample collection, handling and analyticalprocedures for fuel oxygenates, EPA released on a factsheet titled “Analytical Methodologies for Fuel Oxy-genates.” This fact sheet focused on two issues, analyti-cal obstacles, and ether hydrolysis (i.e., MTBE to TBA)when water samples are preserved using acid and heatedfor analysis. EPA recommended the use of a base (such



as trisodium phosphate dodecahydrate) to preserve sam-ples and EPA 8260 as the preferred analytical method for groundwater samples (U.S. EPA, 2003).

OCCURRENCE DATA

The Clean Air Act, Clean Water Act, and Safe Drink-ing Water Act do not require the monitoring of fueloxygenates in air, surface water, groundwater or drink-ing water (Zogorski et al., 1997). As a result, with theexception of MTBE, the environmental occurrence offuel oxygenates is not well documented. This is espe-cially true for EtOH, which has neither a drinking wa-ter standard nor an advisory limit. In addition, there isvery limited occurrence data for ETBE, TAME, andDIPE. TBA occurrence data is increasing of late be-cause of interest in MTBE and its potential biotic break-down to TBA, as well as its hydrolysis to TBA fol-lowing sample preservation with acid (U.S. EPA,2003). TBA is also found as an impurity in MTBE-blended fuels.

Prior to the mid-1990s, occurrence data for MTBE indrinking water sources were only sparsely available. The

interest in MTBE contamination of drinking water sup-plies increased dramatically after several studies were con-ducted to quantify the impact of MTBE contamination ondrinking water supplies nationwide. Early reports sug-gested that the total percentage of wells impacted withMTBE was approximately 5% (Zogorski et al., 1997). Areport issued by U.S. EPA’s Blue Ribbon Panel on Oxy-genates in Gasoline (1999) suggested that 5 to 10% of com-munity drinking water supplies in high MTBE use areashad detectable MTBE, with 1% of those exhibiting MTBEconcentrations exceeding 20 mg/L. In addition, a study es-timated that an upper bound of 1.2% and a lower boundof 0.3% of California’s groundwater supply wells were im-pacted by MTBE (University of California, 1998).

To supplement previously published MTBE occur-rence data, we obtained recent MTBE monitoring datafor drinking water from several states. States were cho-sen based on geographical distribution, the type of exist-ing state regulations and the amount of previously pub-lished occurrence data. MTBE occurrence results fromCalifornia, Massachusetts, and Maryland are presentedfirst, followed by a comparative analysis case study ofMTBE occurrence relative to other oxygenates in South-ern California.

438 DEEB ET AL.

Table 3. Detection of MTBE in public water supply (PWS) sourcesa in California.

1995 1996 1997 1998 1999 2000 2001

Detections in groundwater sources3 , MTBE , 5 mg/L 1 0 1 4 8 5 75 , MTBE , 13 mg/L 0 0 0 3 4 5 5MTBE . 13 mg/L 0 1 3 3 3 3 3Total # detections 1 1 4 10 15 13 15Total detections . 5 mg/L 0 1 3 6 7 8 8Total # sources sampled 89 1,666 2,289 3,151 3,208 2,868 5,248% detection frequencyb 1.12% 0.06% 0.17% 0.32% 0.47% 0.45% 0.29%

Detections in surface water sources3 , MTBE , 5 mg/L 0 1 1 3 6 5 25 , MTBE , 13 mg/L 0 2 5 3 5 3 4MTBE . 13 mg/L 0 0 2 2 0 1 0Total # detections 0 3 8 8 13 9 6Total detections . 5 mg/L 0 2 7 5 5 4 4Total # sources sampled 4 96 176 197 228 251 300% detection frequencyb 0.0% 3.1% 4.5% 4.1% 5.7% 3.4% 2.0%

Mixed/unclassified PWS sourcesc

Total # detections 0 0 0 0 0 0 0Total sources sampled 15 121 162 221 262 248 389

Source: CalDHS database (www.dhs.cahwnet.gov/). aStandby, inactive, destroyed, and agricultural wells were excluded from the analysis, which only addresses drinking water sup-

plies currently in use; bdetection frequency refers to the percent of sampled sources with MTBE concentrations greater than 3 mg/L;csources in the database were either a mixture of groundwater and surface water, or were not easily classified using the available on-line sampling data.

http://www.dhs.cahwnet.gov/

California

Current drinking water standards in California forMTBE are 13 mg/L (primary health-based standard) and5 mg/L (secondary standard, based on taste and odor con-cerns). MTBE detection data in California public drink-ing water sources presented in Table 3 and Figs. 2 and 3were obtained from the California Department of HealthServices database and represent samples collected beforeJanuary 1, 2002. Although some MTBE data was col-lected prior to 1997, California did not require monitor-ing for MTBE until 1997. The data reveal that the num-ber of MTBE detections in California surface andgroundwater sources increase until 1999, after which theylevel off or decrease (Figs. 2 and 3). This trend is alsoobserved when data are presented as a percentage of thenumber of sources sampled (Table 3). One possible ex-planation for the recent decrease in surface water detec-tion is the impact of new regulations such as watershed

protection measures prohibiting the use of two-stroke en-gines on numerous California reservoirs. Sampling pat-terns may also affect the data. For example, althoughMTBE sampling is required in California for public wa-ter supply sources, it is not required annually, and only36% of drinking water sources were sampled for 3 ormore years between 1995 and 2000, while only 1% weresampled annually (Williams, 2001). Further, frequentpreferential sampling of the most susceptible water sys-tems could result in higher reported MTBE detections inthe mid-1990s and an overestimate of the total percentof impacted drinking water sources in the state.

Massachusetts

The state of Massachusetts has a drinking water guide-line for MTBE of 70 mg/L, a secondary standard at 20to 40 mg/L, and a required detection limit of 5 mg/L. Al-though Massachusetts began compiling MTBE data in



Figure 2. Detection of MTBE in California public surface water supply sources.

Figure 3. Detection of MTBE in California public groundwater supply sources.

1993, monitoring was not required until July 1999. Ananalysis of MTBE concentrations in public water supplysystems between 1994 and 2001 is presented in Table 4.Although the total number of MTBE detections peakedin 1999, so has the maximum number of systems sam-pled. Detection occurrences in 2000–2001 indicate that1–2% of sampled public water supplies had detectablelevels of MTBE.

Maryland

Maryland uses the Federal secondary standard of 20mg/L MTBE in drinking water as its action level, and be-gan routine voluntary sampling for MTBE in January1995. A task force was established in February 2000 toreview existing data and to assess the risks associatedwith MTBE in Maryland’s drinking water (www.mde.state.md.us/assets/documents/waste/mtbe_finalreport.pdf).The task force reported that since 1995, MTBE was de-tected in approximately 100 of the 1,200 public watersystems sampled. Thirteen systems had detections greaterthan 20 mg/L, and were taken out of service or had lev-els that dropped below 20 mg/L. Surface water impactswere negligible. A query of the Maryland Water SupplyDatabase produced the results given in Table 5, showingthat in recent years, approximately 2 to 3% of sampledwater systems contain MTBE at concentrations greaterthan 5 mg/L, compared to 1 to 2% of Massachusetts sys-tems. While 0.3 to 0.5% of Massachusetts’s water sys-tems exceed 20 mg/L, 0.5 to 1.0% of Maryland water sys-tems exceed this Federal guideline.

It is important to note that MTBE occurrence data inCalifornia, Massachusetts, and Maryland are not neces-sarily representative of MTBE occurrence in states na-tionwide. However, the data indicate that MTBE detec-tions in drinking water supplies are likely stabilizing infrequency since 1999 in these three states. Decreasing

MTBE detections and concentration trends are likely tobe observed as LUST sites are remediated and as the useof MTBE declines in the future.

MTBE occurrence relative to other fueloxygenates: A case study

Although oxygenates other than MTBE have not beentypical contaminants of concern at LUST sites nationally,monitoring for these compounds has recently been requiredby some agencies. For example, the Los Angeles RegionalWater Quality Control Board (LA RWQCB) routinely re-quires analysis for TBA, TAME, DIPE, ETBE, and EtOHin addition to MTBE at gasoline-impacted sites. Evaluationof groundwater samples collected within approximately 5square miles in West Los Angeles (Charnock drinking wa-ter wellfield) in 2001 revealed MTBE contamination in33% of the samples with a maximum value of 23,000 mg/Land TBA contamination in 22% of samples with a maxi-mum value of 3,400 mg/L (Table 6). These values signifi-cantly exceed the California Public Health Goals for MTBEand TBA of 13 and 12 mg/L, respectively. In contrast,TAME, DIPE, and ETBE were detected in 2.3, 3.4, and0.3% of samples, respectively, at concentrations that neverexceeded 25 mg/L. EtOH was also detected in 2.5% of sam-ples at a concentration of 740 mg/L. The results from thiswellfield study indicate that MTBE and TBA are the pri-mary oxygenates of concern at this site, and that the TBAis of special concern because it is more difficult to removefrom water than MTBE and has low regulatory standardsfor drinking water and discharge.

Another study conducted by LA RWQCB evaluated629 groundwater samples from 73 sites in the Los An-geles area (Rong, 2002). MTBE was detected in 72% ofthe samples, TBA in 33% of samples, TAME in 8% ofsamples, DIPE in 14% of samples, and ETBE in 2% ofsamples (Table 7) with mean MTBE and TBA concen-trations of 7,100 mg/L and maximum concentrations

440 DEEB ET AL.

Table 4. Detection of MTBE in public water supply systems in Massachusetts.

1994 1995 1996 1997 1998 1999 2000 2001

5 , MTBE , 20 mg/L 1 0 1 3 5 17 11 1020 , MTBE , 40 mg/L 2 0 0 1 3 2 1 240 , MTBE , 70 mg/L 1 1 0 1 0 1 2 1MTBE . 70 mg/L 1 1 0 1 1 0 1 1

Total # detectionsa 5 2 1 6 9 20 15 14

Minimum # systems sampled 9 5 12 25 48 94 1363 853Maximum # systems sampled 621 937 900 1122 1148 1395 1363 853

Source: Data obtained from electronic database provided by Massachusetts Drinking Water Program.aDetections are defined as all reported values greater than 5 mg/L, the reporting limit required by the state.

http://www.mde.state.md.us/assets/documents/waste/mtbe_finalreport.pdfhttp://www.mde.state.md.us/assets/documents/waste/mtbe_finalreport.pdf

above 100,000 mg/L. In this wider area study, MTBE andTBA were again the primary contaminants of concern;however, TAME and DIPE were found at high enoughconcentrations to warrant concern.

TREATMENT

The hydrophilic nature of MTBE, coupled with its de-tection in groundwater at LUST sites nationwide, hascaused environmental cleanup managers and regulatorsto reassess cleanup strategies at gasoline-impacted sites.Prior to the emergence of oxygenates at LUST sites, thetypical remedial strategy involved the removal of freeproduct and soil remediation to eliminate continuedsources of gasoline releases, followed by either activeplume remediation or reliance on natural attenuation. Theoccurrence of MTBE at LUST sites prompts a numberof concerns regarding the feasibility and cost-effective-

ness of soil and groundwater remediation, includingdoubts about whether natural attenuation could be ex-ploited as a component of site cleanup strategies. The fol-lowing sections discuss the potential for success of ac-tive and passive remedial strategies at MTBE-impactedsites.

Active remedial approaches at MTBE-impacted sites

Many of the conventional technologies used for reme-diation of gasoline-contaminated sites have been shownto be effective at removing MTBE from soil and ground-water, albeit with less efficiency and higher cost (Bruceet al., 1998; Creek and Davidson, 1998; Mortensenn etal., 2000). Removal of MTBE from soils by soil vaporextraction and multiphase high-vacuum extraction havebeen widely used for LUST cleanups while removal ofMTBE from aquifers using conventional pump-and-treat



Table 5. Detection of MTBE in public water supply systems in Maryland.

1995 1996 1997 1998 1999 2000 2001

3 , MTBE , 5 mg/L 0 2 17 19 15 27 32 5 , MTBE , 20 mg/L 2 7 10 8 5 5 8MTBE . 20 mg/L 2 3 3 3 4 5 3

Total # detections . 3 mg/L 4 12 30 30 24 37 43Total # detections . 5 mg/L 4 10 13 11 9 10 11Total # systems sampleda 940 596 555 659 388 384 323

% Detection frequency (.3 mg/L) 0.4% 2.0% 5.4% 4.6% 6.2% 9.6% 13.3%% Detection frequency (.5 mg/L) 0.4% 1.7% 2.3% 1.7% 2.3% 2.6% 3.4%

Source: Data obtained from electronic database provided by Maryland Water Supply Program.aA subset of the total 1,203 public water systems were tested each year.

Table 6. Fuel oxygenates detected in groundwater samples from the Charnock Wellfield site, California.

MTBE TBA TAME DIPE ETBE Ethanol

2001 # Samples analyzed 1,616 1,562 1,558 1,553 1,558 909# Samples with detections 529 345 36 53 4 23% Detections 32.7% 22.1% 2.3% 3.4% 0.3% 2.5%Maximum concentration (mg/L) 23,090 3,400 24.1 52 4.81 740

2000 # Samples analyzed 1,804 1,647 1,594 1,527 1,595 83# Samples with detections 494 270 28 74 5 1% Detections 27.4% 16.4% 1.8% 4.8% 0.3% 1.2%Maximum concentration (mg/L) 43,000 8,100 85.a 10.a 21 270.a

1999 # Samples analyzed 1,244 979 966 966 965 3# Samples with detections 425 163 29 51 13 0% Detections 34.2% 16.6% 3.0% 5.3% 1.3% 0.0%Maximum concentration (mg/L) 156,000 19,000 70 8.3 22 N/.A

Source: Data obtained from database submitted to Los Angeles Regional Water Quality Control Board as of September 9, 2002.aEstimated detection above MDL.

technologies or in situ air sparging has been shown to beeffective (California MTBE Research Partnership, 2003).The success of air stripping and granular activated car-bon in removing MTBE from groundwater as a compo-nent of pump-and-treat has also been documented (Cal-ifornia MTBE Research Partnership, 2000, 2002; Deebet al., 2001b).

In addition to conventional technologies, severalemerging in situ and ex situ technologies have showngreat promise in field studies for MTBE removal fromcontaminated media. Promising in situ technologies in-clude chemical oxidation (U.S. EPA, 1998b; Leethem,2001), bioremediation/bioaugmentation (Salanitro et al.,2000; Steffan et al., 2001; Wilson et al., 2002) and phy-toremediation (Burken and Schnoor, 1998; Newman etal., 2000; Rubin and Ramaswami, 2001; Zhang et al.,2001) while ex situ technologies include sorption on syn-thetic resins (Annesini et al., 2000; California MTBE Re-search Partnership, 2000; Davis and Powers, 2000), ad-vanced oxidation processes (Kang et al., 1999; Cater etal., 2000; Chang and Young, 2000; Stefan et al., 2000;Acreo et al., 2001; Safarzadeh-Amiri, 2001; MalcolmPirnie, Inc., 2003) and biological treatment (Yeh and No-vak, 1995; Fortin and Deshusses, 1999a, 1999b; Acuna-Askar et al., 2000; Deeb et al., 2000a, 2000b, 2001a;Salanitro et al., 2000; Stocking et al., 2000; U.S. EPA,2000; Bradley et al., 2001a, 2001b; Finneran and Lovely,2001; Hatzinger et al., 2001; Landmeyer et al., 2001;Steffan et al., 2001; Dupasquier et al., 2002). Otherpromising ex situ technologies that have not yet beentested at the field level include the use of membranes orsolvent extraction for the removal of MTBE fromgroundwater (Choi et al., 2000; Keller and Bierwagen,2001; Vane et al., 2001).

The presence of TBA in groundwater, either as an im-purity in MTBE-blended fuels or as a biodegradationbyproduct of MTBE, is of concern regarding the effec-tiveness and cost of water treatment and/or site remedi-ation. This is mainly due to the fact that TBA is highlypolar, is only weakly sorbed to activated carbon, and can-

not be removed from water by conventional (standardtemperature and pressure) air stripping technologies.However, TBA can be removed from water in granularactivated carbon and other systems as a result ofbiodegradation. For example, the success of biologicallyactivated carbon has been recently reported by severalvendors including US Microbics (www.bugsatwork.com/SSWM/Bac.pdf). In addition, TBA can also be re-moved from water using advanced oxidation and reverseosmosis.

The selection of a remedial strategy at an MTBE-im-pacted site depends on the volume of contaminated soiland groundwater, contaminant concentration and mass,site-specific hydrogeological characteristics, and risk-based or regulatory-driven cleanup objectives dependingon land use and aquifer designation. As with any reme-dial effort, source removal is critical for the success ofremediation systems at MTBE-impacted sites. Due to thehydrophilic nature of MTBE, rapid responses to releasesof MTBE-containing gasoline are essential to minimizesite characterization and remediation costs.

Potential for success of natural attenuation atMTBE-impacted sites

Some recent field investigations have indicated thatbiodegradation of MTBE in the environment may occurnaturally. In some cases, the addition of oxygen to anoxicenvironments led to measurable increases in thebiodegradation rate of MTBE in groundwater (Salanitroet al., 2000; Wilson et al., 2002). A first-order biologi-cal decay constant for MTBE following the introductionof oxygen was reported to be 0.008 per day (giving ahalf-life of 87 days), suggesting that biodegradation couldbe a significant mechanism for MTBE attenuation (Salan-itro et al., 2000). However, the cooccurrence of gasolinearomatics has been shown to slow the biodegradation rateof MTBE (Deeb et al., 2001a), suggesting that MTBEcan be most effectively degraded after it has migrated be-yond other gasoline constituents in groundwater.

442 DEEB ET AL.

Table 7. Occurrence of fuel oxygenates at 73 LUST sites in the Los Angeles area.

MTBE TBA TAME DIPE ETBE

# Samples 629 629 629 629 629# Detections 452 211 50 86 14% Detections 72.% 33.% 8.% 14.% 2.%Maximum concentration (mg/L) 550,000 130,000 3,660 3,500 43Mean concentration (mg/L) 7100 7105 178 365 11Median concentration (mg/L) 320 590 17 17.5 3

Source: Data of Los Angeles Regional Water Quality Control Board (adapted from Rong, 2002).

http://www.bugsatwork.com/SSWM/Bac.pdfhttp://www.bugsatwork.com/SSWM/Bac.pdf

In addition to the presence of oxygen, the natural at-tenuation of an MTBE release at a site could potentiallybe enhanced if the indigenous microbial population at thesite has had prior exposure to MTBE and has thereforedeveloped the mechanisms to biodegrade it. Geochemi-cal conditions in the saturated zone, including the pres-ence of electron acceptors and nutrients, as well as fa-vorable pH, also play an important role in determiningthe success of natural attenuation (Bradley et al., 2001a,2001b).

Considering the observed persistence of MTBE in sub-surface environments around the country, it is unlikelythat natural attenuation would be an acceptable sole rem-edy at most MTBE-impacted sites. While it is apparentthat MTBE can biodegrade under both aerobic and an-aerobic (iron-reducing, sulfate-reducing, methanogenic)conditions, the significant biodegradation of MTBE inaquifers has not be commonly observed. Thus, it is im-portant not to extrapolate laboratory MTBE degradationrates to the field, especially when estimating whetherdegradation will be rapid enough to sustain significantplume shrinkage over time. Finally, although the poten-tial for success of intrinsic biodegradation as well as otherattenuation mechanisms is extremely site-specific, in cer-tain hydrogeologic settings (flat gradients, groundwaterflow rates less than 0.1 foot per day), natural attenuationwithout active remediation may be a feasible alternativefor MTBE remediation.

FUTURE RESEARCH NEEDS

Relative to other emerging drinking water contami-nants such as 1,4-dioxane and N-nitrosodimethylamine,there exists a significant body of knowledge regardingthe environmental fate, transport, sources, occurrence,and treatment of fuel oxygenates. However, even thoughmuch has been learned about MTBE and alternative oxy-genates over the last decade, there are still knowledgegaps that can be characterized as research needs. Perhapsone of the most important emerging issues related to oxy-genates is the validity of the analytical data generated todate. Even though early problems and uncertainties as-sociated with analytical methods have been largely over-come, the extent of ether hydrolysis to alcohols follow-ing the preservation of groundwater samples with acidremains unknown. Currently, EPA Method 8260 speci-fies preservation of groundwater samples by the additionof hydrochloric acid. Under acidic conditions and ele-vated temperatures, ether bonds may be hydrolyzed lead-ing to alcohol formation. As a consequence, ether con-centrations (e.g., MTBE) in groundwater samples may beunderestimated while alcohol concentrations (e.g., TBA)

may be overestimated. Because MTBE is often the onlyfuel oxygenate monitored at LUST sites, ether hydroly-sis of groundwater samples could potentially mask thetrue impact of oxygenate contamination and the persis-tence of these compounds in aquifers.

Another important knowledge gap is associated withtreatment technologies for the removal of MTBE andother oxygenates from water. Ex situ treatment of MTBE-contaminated water has been studied extensively withparticular focus on granular activated carbon, air strip-ping, and advanced oxidation processes. Although mostof these technologies have been shown to be successfulfor MTBE removal and are widely used to treat drinkingwater, most of them are not as effective at removing TBA.Although biodegradation is a promising technology forTBA removal, biologically based treatment methods arenot widely accepted for drinking water applications.Hence, one of the research needs associated with oxy-genate contamination is a comprehensive evaluation oftreatment options for TBA-contaminated water, and ap-proaches to get regulatory approval and consumer ac-ceptance for the use of biological processes for the treat-ment of TBA-contaminated drinking waters.

With regards to in situ treatment technologies, resultsfrom bioremediation demonstrations have shown greatpromise at MTBE- and TBA-impacted sites. However,several research gaps still exist regarding the applicabil-ity of biological approaches at oxygenate-contaminatedsites. Although great advances have been made in our un-derstanding of both metabolic and cometabolic MTBEdegrading processes, there is a need for better knowledgeof how to optimize these processes in the field. In addi-tion, while bioaugmentation with MTBE degraders hasbeen shown to be successful in the field, more researchis needed to understand when this activity is necessaryand when oxygen or other nutrient supplements are ben-eficial. In addition, very little is known about the anaer-obic biodegradation of MTBE and TBA and micro-or-ganisms have yet to be isolated with these capabilities,making it difficult to speculate on the nature of the bio-chemical mechanisms responsible for oxygenate degra-dation and to optimize these degradation capabilities. Thestate of knowledge regarding the use of in situ bioreme-diation at MTBE- and TBA-impacted sites is currentlylimited to aerobic applications. It is clear that oxygen de-livery is not effective in many environments, especiallywhen conditions in aquifers are very reduced. Highly re-ducing conditions are typical of source areas at gasoline-impacted sites.

Prior to the discovery of MTBE at gasoline-contami-nated sites, monitored natural attenuation was heavily re-lied on as a remedial approach as many LUST sites. Withthe discovery of MTBE and TBA at many sites, a more



active remedial approach has been required. Perhaps oneof the greatest research needs related to MTBE and TBAcontamination is the applicability of natural attenuationat MTBE-impacted sites either as a stand-alone strategyor as a component of an active remedial approach. Oneof the most critical components of natural attenuation isbiodegradation, and there are still several knowledge gapsassociated with MTBE and TBA biodegradation. Someof these include the need to better understand the factorslimiting MTBE degradation in the field, the occurrenceof indigenous MTBE-degrading organisms in aquifers,and finally, the development of tools to estimate and val-idate microbial MTBE biodegradation rates in the field.

In closing, several questions arise regarding the pend-ing phaseout of MTBE and the introduction of ethanolas another high-volume oxygenate in gasoline. AlthoughEtOH’s properties vary significantly from those ofMTBE, and although EtOH’s environmental impacts areexpected to be different compared to those of MTBE,some recent studies suggest that the use of EtOH mayhave some negative impacts on groundwater quality(Deeb et al., 2002, and references therein). One lessonlearned from recent events associated with MTBE is theneed for comprehensive life-cycle environmental impactstudies prior to the use of any chemical in large quanti-ties. Ideally, current scientific knowledge should influ-ence policy makers to avoid replacing one environmen-tal problem with another. In any case, regardless of whichoxygenate is used in gasoline, due to the hazardous na-ture of the components of this fuel, improved efforts areneeded to prevent future releases of gasoline into the en-vironment.

ACKNOWLEDGMENTS

The authors acknowledge the assistance of MarylineLaugier and Elisabeth Hawley of Malcolm Pirnie, Inc.,in obtaining and analyzing MTBE occurrence data.

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