National Groundwater andContaminated Land Centre

The fueladditive MTBE-a groundwaterprotectionissue?

Introduction

Common additives to petrol worldwide are Methyl Tertiary Butyl Ether,better known as MTBE and its sistercompounds TAME (Tertiary AmylMethyl Ether) and ETBE (EthylTertiary Butyl Ether. Oxylate etherssuch as MTBE were originally addedto replace lead in petrol as "anti-knock" agents. Ethers are now seenby some to have a much broader rolein reducing polluting emissions fromcars. Industry and regulatory bodies,particularly in the USA, see ethers asthe best way to reduce environmentalimpacts of petrol use. By providingextra oxygen for the combustionprocess, ethers reduce the amounts ofun-burnt hydrocarbons in exhaustemissions. In addition, they substitutefor benzene and other aromatics inpetrol and so help reduce publicexposure to these potentialcarcinogens.

World wide production of MTBE isincreasing, and greater use of thischemical can have both positive andnegative impacts on the environment.In 1996, the Environment Agencycommissioned a research project tobetter understand the issues andenvironmental pressures (particularlyon groundwater) associated with the

use of MTBE. The findings of thisstudy were published in an AgencyR&D Technical Report (P11) entitled'Threat to Potable GroundwaterSupplies from the Fuel AdditiveMTBE'. Since the publication of thisreport, the debate over theenvironmental risks and benefits ofgreater use of MTBE has increased,particularly in the USA. Although useof MTBE is relatively low in the UK,this may change and it is necessary tounderstand how MTBE will impact onthe UK environment and to learn fromexperiences in the USA.

Evidence suggests that the averageBritish consumer buys petrol on price.Suppliers therefore formulate petrol tohave the requisite octane rating, usingthe most cost effective additives. UKrefiners currently add between 0% and10% MTBE, by volume, but arelegally entitled to use up to 15% inleaded or unleaded petrol. As MTBEis relatively expensive, it is onlyadded in significant quantities tocrease Super Unleaded grades ofpetrol, where the high octane ratingcan only be met using ether or alcoholadditives. The octane levels requiredin leaded and un-leaded petrol are met

by mixing the basic blend petrol withhigh-octane petrol.

In the USA, there are strict legislativecontrols restricting lubricants added topetrol. High aromatic blending stockshave been reduced in favour of etherand alcohol additives. These increasethe oxygen content of the petroleumand reduces volatility and emissionson combustion. In addition tolegislation, fiscal incentives in theUSA have also promoted the use ofMTBE and methanol to reduce thearomatic content of petroleum. In theEU, however, there are no taxsubsidies at present for usingoxygenates in place of the higharomatic blend stocks.

In recent years, a number of pollutionincidents have been identified in theUSA and the UK where petrol (orgasoline) containing MTBE hascontaminated groundwater. Thesepollution incidents may represent onlya small fraction of the totalgroundwater polluted with MTBE.

One of the reasons for this is thecomparatively recent appearance ofMTBE in both the UK and the USA

markets. It is likely therefore thatmany discharges into groundwaterhave not yet been detected as theyhave not yet reached receptors such aswater supply boreholes, at whichroutine groundwater qualitymonitoring is undertaken. To datemost site investigations and riskassessments carried out at sitescontaminated with petrol haveconcentrated on the detection andquantification of BTEX compounds(Benzene, Toluene, Ethyl-benzeneand Xylene) and Total PetroleumHydrocarbons (TPH). Fewinvestigations target MTBE and manylaboratories are not equipped fourroutine analysis of MTBE.

In March 1999 California announcedplans to phase out MTBE use by2003. This followed a number of highprofile incidents involving the loss ofgroundwater supplies due to MTBEpollution.

Properties of MTBE

Physical & ChemicalProperties

Fate and transport

The movement of MTBE and otherpetroleum components in soil andgroundwater will be controlled bynumerous site-specificenvironmental conditions. Thesemay include the quantity andduration of the petroleum release,soil type, depth to the water table,redox potential, pH, oxygenconcentration, nutrientconcentrations available tofacilitate biodegradation, ambienttemperature and rainfall. A largevolume of petroleum leaking into

The soil is likely to result inpollutants migrating downward tothe water table. During thismigration through the unsaturatedzone, the various components inthe petroleum may be subject tobiodegradation, soil sorption,volatilisation, dispersion anddissolution.

MTBE being less dense thanwater, with a specific gravity of0.74, will float on groundwater.However, MTBE can enter deepgroundwater in response to naturalor induced hydraulic gradients anddiffuse recharge or by moleculardiffusion and dispersion. The mostlikely causes of MTBE in deepgroundwater is steep verticalgradients caused by extractionwells or where vertical hydraulicgradients occur naturally.

Solubility and sorption

MTBE is highly soluble in water,having a solubility of an order ofmagnitude greater than othercomponents of petroleum. Unlikemany other organic chemicals,MTBE is poorly sorbed to carbonbased substrates such as soil.These two physical propertieshave important consequences forthe movement of MTBE ingroundwater and the types ofremediation technology that arelikely to be effective in removing itfrom contaminated groundwater.

(Bio)degradation

The degradation of an organiccompound refers to itstransformation to simpler chemicalcomponents by biotic and abioticreactions. Biologicaltransformations or biodegradationoften provides the predominantdecay pathways for the breakdown

of a wide range of organiccompounds in water and soil.There are relatively few publishedstudies on the biodegradability ofMTBE. Most early studiesindicated that MTBE does notbiodegrade rapidly in theenvironment and it was oftenreported as being recalcitrant.This is in part due to MTBE beinga relatively new contaminant in theenvironment and as a result therehas been limited evolutionarypressure for aquifer micro-organisms to develop degradativecapabilities for this compound.Nevertheless, bacteria have beenisolated which are capable ofdegrading MTBE and recentlypublished experimental findings ofaerobic MTBE biodegradation atthe Borden aquifer in Canadaindicate that naturally occurringbiological process may beimportant in degrading MTBE(Schirmer, M & Barker, J. 1998).

Human toxicity

Human toxicological data derivedfrom both inhalation and oralroutes of exposure suggest that

MTBE is of low acute andsubacute toxicity. It does notappear to be either a reproductivetoxicant or to be mutagenic. Thereis ambiguous data relating to thecarcinogenicity of MTBE. Despitethe current scientific debate, itappears that the threshold tasteand odour concentrations of MTBE(around 15µg 1-1) is likely to belower than any future humanhealth-based guidelines value.

LegislationLegislation currently exists in bothEurope and the USA controllingthe use of MTBE. There is afundamental difference inapproach, however, which resultedin the USA broadly adoptinglegislation to encourage the use ofMTBE, while in Europe legislationis in place to limit its use. Bothregions also have a number oflaws that indirectly influence theuse and environmental impact ofMTBE, such as air qualitystandards and controls on oilstorage.

Legislation in theUK and EuropeLegislative Controls on theuse of MTBE in the EU

After consultation with the motorand oil industries, the EuropeanCommission approved a Directiveon the use of fuel oxygenates inDecember 1985. The Directivecame into force in January 1988(EU Directive on fuel oxygenates.1/1/1988).

The following restrictions apply tothe use of ether oxylates in bothleaded and unleaded petrol:

• no more than 10% by volumein petrol freely transportedover borders;

• no more than 15% by volumein petrol in any member statewithout supplementarylabeling: above this volumelabeling is mandatory.

Air quality legislation in the EUAlthough MTBE demand in Europeis growing, the octane problem isnot as great in the USA. This isbecause restrictions on blending

components are not as severe butthis is set to change with a new EUdirective on fuel quality and vehicleemissions. The draft directivestems from the Auto/Oilprogramme, a tri-partite initiativebetween the EuropeanCommission and the oil and carindustries designed to secure cost-effective compliance with airquality standards by 2010. TheEuropean Parliament hastightened up the original proposalsand requires binding vehicleemission and fuel qualitystandards by 2005. The Europeanparliament also voted for tighterfuel quality standards by 2000.Negotiations are still continuing,but once the directive has beenfinalised it will be for memberstates to determine how to achievethese new standards.

Air quality legislation inthe UK

The UK Legislation on air qualitystems from air qualitymanagement provisions in theEnvironment Act 1995.Regulations following on from theAct have now been made whichset out air quality objectives forseven pollutants. Local authorities

will be required to review airquality in their areas and assesswhether those targets are likely tobe achieved by 2005. Where theyare unlikely to be met, theAuthority must designate an AirQuality Management Area(AWQA) and prepare an actionplan setting out measures toensure compliance. The UK'sstrategy for managing air quality isset out in the 'National Air QualityStrategy' document. Part of thestrategy to improve air quality,particularly in urban areas, is toencourage the improvement invehicle and fuel technology. Thestrategy makes specific referenceto encouraging the use of lesspolluting fuels, but does notspecify what form these fuelsmight take.

Petroleum (Consolidation)Act 1928The keeping of petrol is coveredby the Petroleum (Consolidation)Act 1928. This Act requires thatthe keeping of petrol must beauthorised by a licence. TheHealth and Safety Executive(HSE) has policy responsibilities,but the enforcement and licenceissue role, and setting of anyconditions attached to it, are the

responsibility of the PetroleumLicensing Authorities (PLAs).

The PLAs, which are normallywithin the Fire and Civil DefenceAuthorities, Local Authorities or,under certain circumstances, theHSE are responsible forregulating:

• the issue of licence andlicence conditions under thePetroleum (Consolidation) Act1928, and;

• the loading of petrol from roadtankers at licensed premises.

Water Resources Act 1991

Under the Water Resources Act1991 it is an offence to '... cause orknowingly permit any poisonous,noxious or polluting matter .... toenter any controlled waters'. It isnot necessary for an off-siteimpact to be caused for an offenceto have been committed sincegroundwater, including that directlybeneath a site, is classed ascontrolled water under the Act.

Groundwater Regulations1998

The UK Environment Agencieshave new powers under theGroundwater Regulations 1998,which are necessary to fullytranspose the EU GroundwaterDirective (80/68/EEC) into UK law.This may have a significant impacton the storage of petrol in the UKbecause it requires that all List 1substances (which includehydrocarbons and mineral oils) areprevented from enteringgroundwater. Some estimatessuggest that up to one third of allretail service stations may havepollution problems, so there islikely to be a need to reassesshow petrol is stored in manysituations.

(Proposed) Oil StorageRegulations

The Department of theEnvironment, Transport and theRegions (DETR) is consideringintroducing regulations to setminimum standards for industrialand commercial oil storagefacilities. The regulations wouldapply to all existing installationsand those constructed or

substantially reconstructed orextended after the regulationscome into effect. The mainprovisions would concern designstandards for oil storage tanks andcontainment systems. At existinglocations, powers would be givento the Environment Agency toserve notice requiring action toimprove any installation that itconsiders poses a significant riskof causing water pollution.

Legislation in theUSA

MTBE used to improve airquality

In 1996, 80% of the world'sproduction of MTBE was used inthe USA. The demand for oxygenadditives in the USA is largelydriven by regulations. Emissionstandards imposed by the 1990Clean Air Act Amendments (CAA)are expected to requireoxygenates in over 70% of theUSA petroleum pool by 2000. Thefirst phase of the CAA involves 44areas of the country that failed theUS Environmental ProtectionAgency (US EPA) emissionstandards for carbon monoxide.

From November 1992 retailers inthese areas were committed toselling petroleum containing atleast 2.7% oxygen (by weight) inthe winter months; this part of theCAA is referred to as theOxygenated PetroleumProgramme. In March 1999California announced a plan to endMTBE use in the state by 2003and asked the federal EPA towaive the oxygenate requirementsof CAA.

Oil storage legislation inthe USA

The US EPA proposed extensiveregulations in April 1987 requiringthat all new Underground StorageTanks (USTs) meet an updatedspecification. Within 3-5 years ofthe regulations coming into forceall tanks were required to havelead and overflow detectiondevices. All existing USTs were tobe upgraded or replaced within tenyears. In addition, the US EPAhas developed a series ofstandard test procedures for leakdetection. These were developedto provide a consistent andrigorous evaluation procedure fordetermining the performanceofleak detection methods.

Liability insurance is also requiredin the USA for all owners ofpetroleum USTs at production,refining marketing facilities or sitesthat handle over 10,000 gallonsper month. This insurance coversaccidental spillage and can beused to pay for remediation ofpollution.

Drinking Water Standardsand Regulations in USA

The USA has a variety ofstandards for MTBE in drinkingwater, but only in Florida are theylegally binding.

State Drinking WaterStandards

Florida 50µg 1-1

State Drinking WaterGuidelines (US EPA 1993)

Connecticut 100µg 1-1

Massachusetts 50µg 1-1

Maine 50µg 1-1

Michigan 40µg 1-1

New Hampshire100µg 1-1

New Jersey 700µg 1-1

US EPA standards released in1997: advisory limits of 20-40µg 1-

1

There are no directly equivalentstandards to MTBE in drinkingwater in the UK.

Monitoringgroundwater forMTBE

There is insufficient data atpresent to make any accurateassessment of the current riskposed by MTBE to UKgroundwater resources. So far,there are few reported casestudies of groundwatercontamination by MTBE in Europeand the UK. Very little routinemonitoring data of MTBE in eitherdrinking water sources orgroundwater resources areavailable and the data that isavailable has largely come fromspecific pollution incident studies.The lack of data makes itimpossible to assess thewidespread occurrence of MTBEin UK groundwater and likewisethe threat it poses to generalresource quality. Based on thelimited data available, MTBE hasbeen detected in rural, urban andindustrial areas and in a range ofaquifers, albeit at concentrations

less than the taste and odourthreshold. The Agency intends toincrease the monitoring of MTBEin order to improve itsunderstanding of the nature of thischemical in UK groundwater andthe possible interaction withsurface waters.

A survey carried out in 1996showed that 5 out of 8Environment Agency Regions hadrecorded instances of MTBE ingroundwater. However, no specialattention is payed to watersupplies located close topetroleum outlets (where the riskof spillage and leaking tanks ishigh). At the time of the surveyonly one water services companyroutinely monitored for MTBE,although 40% paid particularattention to water supplies locatednear to retail service stations andof these 30% detected MTBE. Amonitoring strategy is beingdeveloped.

During 1993/4, as part of the USGeological Survey's NationalWater Quality Assessment(NAWQA) programme, a sixteenstate survey of pollutants ingroundwater was undertaken. Atotal of sixty volatile organiccompounds (VOCs) were

measured at each location,including MTBE. The programmewas designed to determinebaseline groundwater quality,against which future trends inwater quality within sixty of thelargest and most important riverbasins were to be established.Sampling wells were selected on a'grid based random sampling'approach. MTBE was detected ingroundwater across the studyarea, but was found most often aturban sites (27%) compared tothose in rural locations (1.3%). Ofthe sixty VOCs analysed, MTBEwas the second most frequentlydetected in samples of shallowgroundwater from urban areas. Itwas concluded that a combinationof spills, fugitive emissions, leakingunderground storage tanks andun-burnt petroleum from poorlyoperating vehicles could accountfor the wide distribution of MTBE inthe study locations.

The US EPA gives advice on howto monitor and sample sitesalready contaminated with MTBE.It notes that because MTBEbehaves differently from otherpetroleum hydrocarbons when

released into the environment, asite investigation may need to bemodified to properly characterisethe area of MTBE contamination.It has been observed that therelatively high solubility of MTBE,when combined withheterogeneous groundwater flowbehaviour, may result in the MTBEdissolving into the groundwater inpulses. This results in large andrapid changes in contaminantconcentrations. Pulses, whichmay be caused by groundwaterflow through anisotropic aquifersystems, groundwater headfluctuations or infiltration ofrainwater, may necessitatefrequent groundwater sampling todetermine actual MTBEconcentrations and levels of risk todown-gradient receptors. Thefrequency of sampling should takeaccount of the local hydrogeologyand the number of monitoringwells. Accurate sampling data isessential to determining the impactof MTBE at a particular site.

Case studies

UK case study: EastAnglia

IN 1988, a site in the Agency'sAnglian region was contaminatedby the release of unleaded fuelfrom an underground tank.Groundwater in the chalk aquiferand at a public supply boreholewas affected by the release.'Pump and Treat' and 'air-sparing'technologies were used to mitigatethe effects of the release.Monitoring and testing at the sitewas undertaken which producedthe following conclusions:

• MTBE is persistent ingroundwater by comparisonwith BTEZX (Benzene,Toluene, Ethyl-Benzene andXylenes) components.

• The MTBE plume exhibits littleretardation in comparison toother BTEX components. TheMTBE plume is larger than theBTEX plumes andcontaminates greater volumesof groundwater. This is

consistent with experiences inthe USA.

• The 'Pump and Treat' systemwas only partially effective incontaining aqueous phasecontaminants.

• The 'Pump and Treat' systemwas unsuccessful at removingthe floating (free-phase)petroleum contaminant.

• The effectiveness of 'Pumpand Treat' systems for MTBEmay be limited by the ability ofthe system to recover thecontaminant adsorbed toorganic or clay particles in theaquifer, or held within porespaces as a result of highpore-throat breakthrough(capillary) pressures.

• Vapour extraction techniquesproved the most effectivetechnique at removing thepollutants from the aquifer.

USA case study: California

In 1990, the US Clean Air Actobliged areas with poor air qualityto add oxygenates to their petrol to

increase the oxygen content andimprove combustion, therebyreducing noxious emissions.California is a good examplewhere action has been taken toovercome air pollution but hasresulted in major problems in thewater environment. MTBE hasnow widely contaminatedgroundwater and surface water inCalifornia, primarily as a result ofleaking underground storage tanksand pipelines and the use ofmotorized vehicles on lakes andreservoirs.

In late 1995, authorities in the cityof Santa Monica, Californiadetected MTBE in a well thatsupplied the city with drinkingwater. In June the following yearthe problem had escalated and thecity authorities were forced to shutdown some of the potable supplygroundwater wells. As a result ofthe MTBE contamination, the citylost 71% of its local water supply.About half of its total waterconsumption must now be boughtfrom outside sources at a cost of$3.5 million a year.

Until recently oil companies in theUSA unanimously supported theuse of MTBE, but in October 97,

Tosco, the largest independent oilrefiner in the USA wrote to theCalifornia Air Resources Boardasking them to 'take decisiveaction immediately to begin tomove away from MTBE'. Toscowarned that extensivecontamination would result in hugecosts to restore the state's drinkingwater.

Sources of MTBE pollution

Defining the source of MTBEpollution in groundwater isessential to prevent furthercontamination and to protectvulnerable aquifers. MTBE canenter the environment by eitherpoint or diffuse sources.

Point Sources

MTBE is usually discharged intothe environment as a componentof petroleum. Pollution incidentdata suggests the most importantpoint sources are spills and leaksfrom both underground and aboveground storage tanks, pipelinesand transportation accidents andby disposal of contaminated soil atlandfills, following the excavationand removal of soils fromcontaminated sites.

Diffuse sources

Diffuse sources are also a possiblecause of groundwatercontamination; examples includeatmospheric deposition and stormwater run-off from highways(Squillace et all 1995). MTBE isreleased into the atmosphere froma variety of sources includingindustrial discharges, mobilesources such as emissions frommotor vehicles and also duringrefuelling of vehicles. It issobering to reflect that each time acar petrol tank is refilled witharound 50 litres of petrol then 50litres of vapour are forced out.Few studies have measuredMTBE concentrations in theatmosphere, but recent evidenceindicates evaporative emissionsfrom vehicles are higher than wasoriginally thought, suggesting thismay be an important route forMTBE to enter the environment(Stump 1990, Calvert 1993).

Petroleum distributionsystem

The majority of petroleum (55-60%) is transported from therefinery to the marketing terminalthrough pipelines (Environment

Agency R&D Technical Report11). Around 5-10% is transportedby road tanker, with the remaindershipped from the refinery by bargeand tanker. Petroleum is notcurrently moved in any significantamounts by rail, although rail oftenplays a major role in movingoxygenates to their blendinglocations.

Refineries may blend petroleumcomponents continuously and thendeliver directly to a pipeline, orthey may batch blend thepetroleum into shipping tanks.Long pipelines often haveintermediate breakout tanks thatare necessary for efficientoperation of the pipeline. Mostpipelines have multiple deliverypoints along the line. All theselocations represent potential pointsof failure or long-term leakage.Since MTBE blended petroleum isusually compatible with standardstorage and distribution systems,the blending process usuallyoccurs at the refinery, but MTBEcan also be added at marketingterminals. Release of MTBE intothe environment through spillagegenerally occurs as aconsequence of petroleum spills.

The could include leakageassociated with:

• refinery tanks and receivingfacilities;

• corrosion or damage to fuelstorage and distributionsystems;

• failure of valve and pumpseals associated with pipelinedistribution systems;

• spillages during dispensing;• spillages from tankers during

delivery or transportation

In order to ensure spillage is keptto a minimum, accurate monitoringof leakage is needed at all pointsin the distribution system. A rangeof leak detection and safetysystems could help to minimise theeffects of failure and leakage.These may include:

• storage tanks andunderground lines withsecondary containment andinterstitial monitoring systems;

• tank overfill protection, andunder-pump check valves;

• fill-pipe spill containment (ieappropriate bunds orwatertight man-chambers);

• spillage containment arounddispensers (ie impermeableforecourts with appropriatedrainage);

• the use of appropriatepetroleum resistant materials;

• accurate wet-stockmanagement systems.

Petroleum storage in theUK and Europe

Currently there is no singleapproach to underground storagetank management in Europebecause of differences inenvironmental setting and politicalapproach in the various memberstates. The threat posed togroundwater resources by LeakingUnderground Storage Tanks(LUSTs), has been recognised inEurope for over twenty years,however, there is presently littleuse of external leak detectiontechnology. Where monitoringtechnology has been developed ithas generally focused onassessing accurate stock records,rather than specifically addressingenvironmental protection issues.Sophisticated monitoringequipment is now available tomeasure tank contents and the

presence of fuel in soil andgroundwater outside the storagesystem. These facilities can helpto detect leaks.

Releases at filling stations can beattributed to five main factors:

• customer spills duringdispensing on forecourts,which some estimate ataround 1,000 litres a year persite;

• poorly maintained or specifieddrainage and oil/waterseparators;

• leaks from fuel deliverypipework - experienceindicates that leaks frompipework are the mostcommon problem at retailsites;

• tank leaks of both liquid andvapour;

• overfilling during tankerdeliveries.

The prescriptive regulatoryrequirements for environmentalprotection on the UK oil industryare currently less stringent than inparts of Western Europe and NorthAmerica. Consequently standardsof underground storage installationconstruction and protection, and

associated environmentalmonitoring adopted at many sitesis insufficient to prevent releasesof fuel and pollution of waterresources. Regulation of fuelstorage in the UK hasconcentrated solely on health andsafety issues, associated with theexplosive and fire risks of petrol,and has often neglectedenvironmental considerationsassociated with petrol or other lessflammable fuels such as diesel.

Petroleum storage in theUSA

The US EPA is concerned aboutpotential groundwatercontamination from LUSTs. TheUS EPA expects 75% of all tanksto fail within the next decade. Ithas also estimated that about 22%of the 1,230,000 petroleumstorage tanks at more than500,000 sites in the USA haveleaked as of July 1994.

In 1984, the US EPA compiled areport detailing over 12,000petroleum releases fromUnderground Storage Tanks(USTs). Analysis of the datarevealed some interestingstatistics about petrol spillage:

• reported releases of petroleuminto the environment showed amarked increase since 1974(over 2,500 reported in 1984compared to about 200 in1974);

• most releases were in areaswith relatively corrosive soils(high moisture, low pH);

• 80% of leaks were due tocorrosion or structural failure;

• 84% of leaking tanks werebare steel;

• 11% of leaking tanks wereglass reinforced plastic (GRP);

• 90% of the releases were atretail or other commercialestablishments;

• greater than 40% of the leaksinvolved an excess of 500gallons;

• most leaks (70%) weredetected by sight or smell;

• groundwater wascontaminated in 45% ofreleases;

• more than 700 private wellsand 40 municipal wells werereported as beingcontaminated.

Risk assessmentfor groundwaterpollutionThe UK Government is committedto promoting the redevelopment ofcontaminated land andremediation of groundwater withina risk-based context, takingaccount of land-use andenvironmental setting. Wheregroundwater is, or is likely to becontaminated, risk-based remedialgoals should be developed thatare protective of human health, theenvironment and other relevantreceptors.

The Environment Agency andDETR are currently developingguidance on risk assessmentprocedures and methodologies forthe assessment of risks togroundwater resources from landand groundwater contamination,and for the derivation of site-specific remedial goals in theevent that the risks are deemedunacceptable.

The following guidance is due tobe released by late 1999 (seereferences for further details):

• Model Procedures for theManagement of ContaminatedLand, CLR11: Procedure forRisk Assessment.

• ConSim• Methodology for the Derivation

of Remedial Targets for Soiland Groundwater to ProtectWater Resources.

Comparable approaches havebeen adopted in the USA, such asthe American Society for Testingof Materials' protocol for RiskBased Corrective Action (RBCA).It should be noted that RBCA maynot be directly applicable to the UKwithout modifications to theassumptions made within theprotocol, due to legislativerequirements particular to the UK.

Remediation ofMTBE contaminatedsoils andgroundwaterMTBE can be removed fromgroundwater, along with otherpetroleum components, byconventional remediation methods,although it may increase the cost

and duration of remedial works. Arange of techniques exists forMTBE removal from groundwater(see below), some have provedmore effective though than others.As with all remediationtechnologies site-specificconditions will play a fundamentalrole in the option selectionprocess.

Soil Vapour Extraction

Soil Vapour Extraction (SVE) hasbeen shown to be an effectivetechnology for the remediation ofsoil contaminated with volatile andsemi-volatile organic compounds,such as BTEX and MTBE.

SVE is an in situ soil treatmenttechnology that removes volatilecontaminants from soil in theunsaturated zone (above thewater-table) by extracting thecontaminant vapours with avacuum that is applied to thesubsurface. The contaminatedvapours are then treated toremove the organic compounds(by air-water stripping or granularactivated carbon - GAC) beforebeing vented to atmosphere.

Low Temperature ThermalDesorption

Low Temperature ThermalDesorption (LTTD) is also an insitu soil treatment technology, butin this instance the soil and soilgas are heated to enhancevolatilisation of contaminants fromthe soil matrix. Off-gases must betreated in the same manner asthose produced by a SVEoperation, and it is important tocontrol the temperature to ensurethat ignition of combustiblevapours does not occur.

Air sparging

Air sparging is not particularlyeffective at removing MTBE fromwater. This in situ techniqueinvolves injecting air beneath thewater-table to volatilise thecontaminants. Because MTBEhas a relatively high solubility inwater, larger volumes of air areneed to volatilize MTBE comparedto other petroleum basedpollutants such as the BTEXcompounds. Initial fieldexperiments suggest two to five

times more air is needed to treat agiven volume of water with MTBEcompared to a similar volume ofwater with other VOC compounds,assuming the MTBE concentrationis less than 5,000µg 1-1

'Pump and Treat'

In contrast to many other organicpollutants, pumping contaminatedgroundwater and treating it aboveground ('Pump and Treat') may bea cost-effective remediationtechnology for MTBE. This isbecause MTBE does not adsorbstrongly to soil so removal bypumping groundwater is likely tobe relatively rapid. In addition,because it is highly soluble, mostof the MTBE mass may quicklydissolve into groundwater makingpumping an efficient method ofremoving large quantities ofcontaminant.

Following removal of contaminatedgroundwater, it will need to betreated at the surface (for exampleby air stripping or GAC) beforebeing returned to the ground orbeing discharged to watercourseor sewer. It should be noted,however, that granular activated

carbon is not particularly effectiveat separating MTBE from water.This treatment uses the sorptioncapacity of the activated carbon toadsorb the pollutant. As MTBEhas a very low absorption capacityGAC is only about 1/3 to 1/8 aseffective in removing MTBE as it isin removing benzene.

Monitored NaturalAttenuation

Recent studies of MTBE plumes inNorth America, and in particularthe Borden aquifer of Ontario,Canada have shown that, underpreferential hydrogeochemicalconditions, natural biodegradationof MTBE can occur withingroundwater systems, (Schirmer &Barker, 1998). Degradationkinetics were significantly slowerthan those for the BTEXcompounds. If hydrogeologicalconditions allow, however, andrisk-assessment demonstratesenvironmental impacts areacceptable throughout thepredicted duration of thecontamination, it may beappropriate to manage the risksassociated with an MTBE pollution

by adopting a monitored naturalattenuation strategy.Information on other technologiescan be found in the draftContaminated Land ResearchReport (CLR11: Model Procedurefor the Evaluation and Selection ofRemedial Measures.

At a glance

MTBE:

• is used as an additive inpetrol;

• increases the oxygen contentof petroleum and results inlower aromatic hydrocarbonemission from vehicles, but itmay increase emissions ofNOx and aldehydes;

• is relatively non-toxic tohumans, but can be detectedby taste and odour at lowconcentrations (5-15µg 1-1);

• is more soluble than othercomponents of petrol and, ifspilt, will consequently travel

• further and contaminategreater volumes ofgroundwater than the othercomponents of petrol;

• and other oxygenates haveonly been used in recent yearsand so the full extent ofgroundwater pollution may notbe evident yet.

Since MTBE is relatively slow tobiodegrade, any pollution incidentsnow could impact on the quality ofthe environment for years to come.Consequently whatever decision istaken on the future use of MTBEmust be based on sound scientificinformation and take an integratedenvironmental approach thatincorporates the principles ofsustainability.

• is relatively recalcitrant (incomparison to BTEXcompounds) and will remain inthe environment for extendedperiods;

• can often be detected beyondthe outer fringes of a BTEXplume.

Certain States in the USA,particularly California, have amajor problem with contamination

of drinking waters sources withMTBE. We must learn from theAmerican experience.

Further monitoring is needed toget a better understanding of theextent of the MTBE pollution in theUK. The Environment Agencyintends to extend investigationsinto the presence of MTBE in theenvironment and to assess itsimpact on integrated groundwaterand surface water systems.

References andfurther guidanceConSim Environment Agency1994Produced by: Golders AssociatesUK LimitedLandmere Lane, Edwalton,Nottingham NG 4DG

DETR & Environment Agency,1999Handbook of Model Procedures forthe Management of ContaminatedLandContaminated Land ResearchReport CLR11: Procedure for RiskAssessment (Draft)

DETR & Environment Agency,1998

Handbook of Model Procedures forthe Management of ContaminatedLandContaminated Land ResearchReport CLR11: Procedure for theEvaluation and Selection ofRemedial Measures (Draft).

Environment Agency, 1996Groundwater Pollution: Evaluationof the Extent and Character ofGroundwater Pollution from PointSources in England and Wales.

Environment Agency, 1996R&D Technical Report P11; Threatto Potable Groundwater Suppliesfrom the Fuel Additive MTBE.

Environment Agency 1998.Policy and Practice for theProtection of Groundwater.

Environment Agency, 1999Methodology for the Derivation ofRemedial Targets for Soil andGroundwater to Protect WaterResources. (Draft).R&D Technical Report P186.

United Sates EnvironmentalProtection Agency, 1998.MTBE Fact Sheet #2: Remediationof MTBE Contaminated Soil andGroundwater.

United States Geological Survey.Environmental behaviour and fateof Methyl tert-Butyl Ether (MTBE).Fact Sheet FS-203-96.

Schirmer, M & Barker, J, 1998.A Study of Long-Term MTBEAttenuation in the Border Aquifer,Ontario, Canada. GroundwaterMonitoring & Remediation, Spring1998.

Squillace P.J & Zogorski J. SA Preliminary Assessment of theOccurrence of Possible Sources ofMTBE in Groundwater of theUnited States, 1993-94, UnitedStates Geological Survey, 1995.

Stump FD, Knapp KT 1990.Seasonal impact of oxygenatedorganics with petroleum on motorvehicle tail pipe and evaporativeemissions. Journal of Air & WasteManagement Association V40, no.6.

R&D Technical Report can beobtained from the R&D section ofEnvironment Agency RegionalOffices.

MANAGEMENT AND CONTACTSThe Environment Agency delivers a service to its customers with the emphasis on authority andaccountability at the most local level possible. It aims to be cost-effective and efficient and to offer the bestservice and value for money.

Head Office is responsible for overall policy and relationships with national bodies including Government.Rio House, Waterside Drive, Aztec West, Almondsbury, Bristol BS32 4UD

Internet World Wide Web www.environment-agency.gov.uk

NATIONAL GROUNDWATER & CONTAMINATED LAND CENTREOlton Court, 10 Warwick Road, Olton, Solihull B92 7HXTel 0121 711 5885 Fax 0121 711 5925E-mail ngwclc@environment-agency.gov.uk

ENVIRONMENT AGENCY REGIONAL OFFICESANGLIANKingfisher HouseGoldhay WayOrton GoldhayPeterborough PE25ZRTel: 01733 371 811Fax: 01733 231 840

SOUTHERNGuildbourne HouseChatsworth RoadWorthingWest Sussex BN11 1LDTel: 01903 832 000Fax: 01903 821 832

MIDLANDSSapphire East550 StreetsbrookRoadSolihull B91 1QTTel: 0121 711 2324Fax: 0121 711 5824

SOUTH WESTManley HouseKestrel WayExeter EX2 7LQTel: 01392 444 000Fax: 01392 444 238

NORTH EASTRivers House21 Park Square SouthLeeds LS1 2QGTel: 0113 244 0191Fax: 0113 246 1889

THAMESKings Meadow HouseKings Meadow RoadReading RG1 8DQTel: 0118 953 5000Fax: 0118 950 0388

NORTH WESTRichard FaircloughHouseKnutsford RoadWarrington WA4 1HGTel: 01925 653 999Fax: 01925 415 961

WALESRivers House/ Plas-yr-AfonSt Mellons Business ParkSt MellonsCardiff CF3 0LTTel 01222 770 088Fax 01222 798 555

For general enquiries please call yourlocal Environment Agency office. If you are unsure who to contact, or which is your local office, please call our general enquiry line.ENVIRONMENT AGENCYGENERAL ENQIRY LINE

0645 333 111The 24-hour emergency hotlinenumber for reporting all environmentalincidents relating to air, land and water.ENVIRONMENT AGENCYEMERGENCY HOTLINE

0800 80 70 60