description, properties, and degradation of selected volatile

Prepared in cooperation with the Agency for Toxic Substances and Disease Registry, U.S. Department of Health and Human Services

Description, Properties, and Degradation of Selected Volatile Organic Compounds Detected in Ground Water — A Review of Selected Literature

Open-File Report 2006-1338

U.S. Department of the InteriorU.S. Geological Survey


By Stephen J. Lawrence

Prepared in cooperation with the Agency for Toxic Substances and Disease Registry, U.S. Department of Health and Human Services

Open-File Report 2006–1338

U.S. Department of the InteriorU.S. Geological Survey

U.S. Department of the InteriorDIRK KEMPTHORNE, Secretary

U.S. Geological SurveyMark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2006

This report is a Web-only publication: http://pubs.usgs.gov/ofr/2006/1338/.

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Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report.

Suggested citation:Lawrence, S.J., 2006, Description, properties, and degradation of selected volatile organic compounds detected in ground water — A Review of Selected Literature: Atlanta, Georgia, U. S. Geological Survey, Open-File Report 2006-1338, 62 p., a Web-only publication at http://pubs.usgs.gov/ofr/2006/1338/.

http://pubs.usgs.gov/ofr/2006/1338/

Contents

Abbreviations and Acronyms.......................................................................................................... viConversion Factors ........................................................................................................................... viiAbstract .............................................................................................................................................. 1Introduction........................................................................................................................................ 1

Purpose and Scope ................................................................................................................. 1Available Literature Addressing Volatile Organic Compounds ........................................ 1

Naming Conventions and Descriptions of Volatile Organic Compounds in Ground Water ....................................................................................................................... 3

Sources of Volatile Organic Compounds Detected in Ground Water ........................................ 6Sources of Chlorinated Alkanes ............................................................................................ 6Sources of Chlorinated Alkenes and Benzenes ................................................................. 9Sources of Gasoline Compounds .......................................................................................... 9

BTEX Compounds (Benzene, Toluene, Ethylbenzene, and Xylene) ........................ 10Methyl Tert-butyl Ether .................................................................................................. 10

Basic Properties of Selected Volatile Organic Compounds ...................................................... 10Degradation of Selected Volatile Organic Compounds in Ground Water ............................... 10

Degradation of the Chlorinated Alkanes .............................................................................. 17Abiotic Transformation ................................................................................................... 17Aerobic Biodegradation ................................................................................................ 20Anaerobic Biodegradation ........................................................................................... 20

Degradation of the Chlorinated Alkenes .............................................................................. 21Aerobic Biodegradation ................................................................................................ 21Anaerobic Biodegradation ............................................................................................ 23

Degradation of the Chlorinated Benzenes .......................................................................... 24Degradation of the Gasoline Compounds ............................................................................ 25

Aerobic Biodegradation of BTEX Compounds .......................................................... 25Anaerobic Biodegradation of BTEX Compounds ...................................................... 27Aerobic Biodegradation of Methyl Tert-butyl Ether .................................................. 30Anaerobic Biodegradation of Methyl Tert-butyl Ether ............................................. 31

References Cited............................................................................................................................... 36Glossary .............................................................................................................................................. 50

iii

Figures 1–19. Diagrams Showing— 1. Generic and International Union of Pure and Applied Chemistry naming

conventions for the same aliphatic compound .......................................................... 5 2. Relation between degree of chlorination and anaerobic reductive-

dechlorination, aerobic degradation and sorption onto subsurface material ...... 16 3. Laboratory-derived pathway for the abiotic degradation, anaerobic, and

methanogenic biodegradation of 1,1,2,2-tetrachloroethane; 1,1,2-trichloroethene; and 1,1,2-trichloroethane ........................................................ 18

4. Laboratory-derived pathway for the abiotic, aerobic, and anaerobic biodegradation of 1,1,1-trichloroethane ...................................................................... 19

5. Laboratory-derived pathway for the aerobic biodegradation of 1,2-dichloroethane ........................................................................................................... 20

6. Laboratory-derived pathways for the anaerobic biodegradation of tetrachloromethane (carbon tetrachloride) ................................................................ 21

7. Laboratory-derived pathways for the aerobic biodegradation of trichloroethene ............................................................................................................ 22

8. Laboratory-derived pathway for the anaerobic biodegradation of tetrachloroethene ....................................................................................................... 23

9. Laboratory-derived pathways for the aerobic and anaerobic biodegradation of 1,2,4-trichlorobenzene ................................................................................................ 24

10. Laboratory-derived pathway for the aerobic biodegradation of 1,4-dichlorobenzene ........................................................................................................ 25

11. Laboratory-derived pathway for the aerobic biodegradation of chlorobenzene and 1,2-dichlorobenzene. ................................................................... 26

12. Laboratory-derived pathways for the aerobic biodegradation of benzene, o-, and m-xylene .............................................................................................................. 28

13. Laboratory-derived pathways for the aerobic biodegradation of toluene ............ 29 14. Laboratory-derived pathways for the aerobic biodegradation of p-xylene .......... 30 15. Laboratory-derived pathway for the aerobic biodegradation of ethylbenzene .... 31 16. Field and laboratory-derived pathways for the anaerobic biodegradation of

the BTEX compounds — benzene, toluene, ethylbenzene, and xylene ................... 33 17. Laboratory-derived pathway for the aerobic biodegradation of methyl

tert-butyl ether ................................................................................................................. 34 18. Laboratory-derived pathway for the aerobic biodegradation of m-cresol ............ 35 19. Laboratory-derived pathways for the aerobic biodegradation of styrene ............ 35

iv

Tables 1. List of selected publications providing literature reviews and summaries of

volatile organic compound degradation and behavior in ground water ................ 2 2. Names and synonyms of volatile organic compounds commonly detected in

ground water .................................................................................................................... 4 3. The first four members of the straight-chain alkane series and associated

alkyl radical ....................................................................................................................... 5 4. Volatile organic compounds ranked by those frequently detected in ground

water near landfills and hazardous waste dumps in the United States and the Federal Republic of Germany ........................................................................................ 6

5. Volatile organic compounds detected in regional and national ground-water studies in the United States ........................................................................................... 7

6. Volatile organic compounds detected in ground-water case studies at selected U.S. Department of Defense installations ................................................... 8

7. Major organic compounds in a typical gasoline blend ............................................. 9 8. Henry’s Law constants for selected volatile organic compounds detected in

ground water .................................................................................................................... 11 9. Water-solubility data for selected volatile organic compounds detected in

ground water .................................................................................................................... 12 10. Density of selected volatile organic compounds detected in ground water

compared to the density of water at 20 degrees Celsius .......................................... 13 11. Octanol-water partition coefficients for selected volatile organic compounds

detected in ground water ............................................................................................... 14 12. Soil-sorption partition coefficients for selected volatile organic compounds

detected in ground water ............................................................................................... 15 13. Common abiotic and biotic reactions involving halogenated

aliphatic hydrocarbons ................................................................................................... 16 14. Laboratory half-lifes and by-products of the abiotic degradation (hydrolysis

or dehydrohalogenation) of chlorinated alkane compounds detected in ground water. ................................................................................................................... 18

15. Mean half-life in days for the anaerobic biodegradation of selected chlorinated alkane and alkene compounds ................................................................ 20

16. Laboratory or environmental half-lifes and by-products for the aerobic and anaerobic biodegradation of selected chlorinated benzene compounds detected in ground water ............................................................................................... 27

17. Average half-life for the aerobic biodegradation of the fuel compounds BTEX and methyl tert-butyl ether to carbon dioxide in an uncontaminated and contaminated matrix of aquifer sediments and ground water ......................... 32

18. Mean half-life in days for the anaerobic biodegradation of the fuel compounds BTEX, and methyl tert-butyl ether, tert-butyl alcohol under various reducing conditions .......................................................................................... 32

v

Abbreviations and Acronyms

ATSDR AgencyforToxicSubstancesandDiseaseRegistryBTEX benzene,toluene,ethylbenzene,xylenesCA chloroethaneCAA CleanAirActCB chlorobenzeneCERCLA ComprehensiveEnvironmentalResponse,Compensation,andLiabilityActCDC CentersforDiseaseControlCO

2 carbondioxide

CSIA compound-specificisotopeanalysisCTET carbontetrachlorideCVOC chlorinatedvolatileorganiccompoundsDCA dichloroethane11-DCA 1,1-dichloroethane12-DCA 1,2-dichloroethane12-DCB 1,2-dichlorobenzene13-DCB 1,3-dichlorobenzene14-DCB 1,4-dichlorobenzeneDCE dichloroethene12-cDCE cis-1,2-dichloroethene12-tDCE trans-1,2-dichloroetheneDNA deoxyribonucleicacidDO dissolvedoxygenFDA U.S.FoodandDrugAdministrationgVOC volatilegasolinecompoundsH Henry’sLawconstantIUC InternationalUnionofChemistryIUPAC InternationalUnionofPureandAppliedChemistryK

oc soilorganiccarbonpartitioncoefficient

Kow

octanol-waterpartitioncoefficientMTBE methyltert-butyletherNAPL non-aqueousphaseliquidORD OfficeofResearchandDevelopmentPCA tetrachloroethanePCE tetrachloroethenePLFA phospholipidfattyacidRCRA ResourceConservationandRecoveryActRFG ReformulatedGasolineprogramRNA ribonucleicacidSMA signaturemetabolitesanalysisTBA tert-butylalcoholTCA trichloroethane112-TCA 1,1,2-trichloroethaneTCB trichlorobenzene123-TCB 1,2,3-trichlorobenzene124-TCB 1,2,4-trichlorobenzeneTCE 1,1,2-trichloroetheneTEA terminalelectronacceptor124-TMB 1,2,4-trimethylbenzeneUSEPA U.S.EnvironmentalProtectionAgencyUSGS U.S.GeologicalSurveyVC vinylchlorideVOC(s) volatileorganiccompound(s)

vi

Conversion Factors

Multiply By To obtain

microgramperliter(µg/L) 6.243108 poundpercubicfoot

microgramperliter(µg/L) 1x10–3 milligramperliter(mg/L)

milligramperliter(mg/L) 6.243105 poundpercubicfoot

grammolepercubicmeter(gmol/m3) 6.243×105 poundpercubicfoot

kiloPascal(kPa) 9.869210–3 standardatmosphere

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) or micrograms per liter (µg/L).

vii


By Stephen J. Lawrence

AbstractThisreportprovidesabridgedinformationdescribing

themostsalientpropertiesandbiodegradationof27chlori-natedvolatileorganiccompoundsdetectedduringground-waterstudiesintheUnitedStates.Thisinformationiscon-densedfromanextensivelistofreports,papers,andliteraturepublishedbytheU.S.Government,variousStategovern-ments,andpeer-reviewedjournals.Thelistincludesliteraturereviews,compilations,andsummariesdescribingvolatileorganiccompoundsingroundwater.Thisreportcross-referencescommonnamesandsynonymsassociatedwithvolatileorganiccompoundswiththenamingconventionssupportedbytheInternationalUnionofPureandAppliedChemistry.Inaddition,thereportdescribesbasicphysicalcharacteristicsofthosecompoundssuchasHenry’sLawconstant,watersolubility,density,octanol-waterpartition(logK

ow),andorganiccarbonpartition(logK

oc)coefficients.

Descriptionsandillustrationsareprovidedfornaturalandlaboratorybiodegradationrates,chemicalby-products,anddegradationpathways.

IntroductionThepresenceofvolatileorganiccompounds(VOCs)

ingroundwaterisamajorconcerntoallwhousegroundwaterasadrinkingwatersourcebecausemanyofthesecompoundscanadverselyaffecthumanhealth.Likewise,concernaboutVOCsingroundwaterissharedbyStateandFederalregulatoryagenciesresponsibleforprotectingtheground-waterresourcefromcontaminantsandforprotect-inghumanhealth.ThisreportispreparedincooperationwithTheAgencyforToxicSubstancesandDiseaseRegistry,U.S.DepartmentofHealthandHumanServices(ATSDR)andprovidesunderonecoveranabridgeddescriptionofselectedproperties,andbiodegradationinformationpublishedinaca-demicandgovernmentliterature.Inaddition,thereportcrossreferencescommonlyusednameswithagenerallyacceptedinternationalnamingconventionfor27VOCsfrequentlydetectedingroundwater.

Purpose and Scope

Thepurposeofthisreportisto(1)list27VOCsfre-quentlydetectedingroundwater,(2)cross-referencecom-monVOCnamesandsynonymsassociatedwiththenamingconventionssupportedbytheInternationalUnionofPureandAppliedChemistry (IUPAC),(2)describethebasicchemicalpropertiesofselectedVOCsbysubclass,and(3)describethevariouspathwaysandchemicalby-productsassociatedwiththedegradationofselectedVOCsingroundwater.ThegoalofthereportisnottosupplantpreviouslypublishedliteraturereviewsonVOCsingroundwater,butrathertocondensethatinformation,andinformationfromotherpapers,intoa“digest”orabridgeddocumentthatdescribesonlythemostsalientandgenerallyacceptedscientificinformationregardingnomenclature,properties,anddegradationpathsfor27VOCsdetectedingroundwaterintheUnitedStates.

Available Literature Addressing Volatile Organic Compounds

Theinformationforthisreportiscondensedfromselectedacademicandgovernmentliteraturepublishedwithinthelast30years(1975–2006)thatdescribelaboratoryandfieldexperimentsandground-waterstudiesofVOC.The27VOCsdescribedinthisreportareamongtheVOCscom-monlydetectedinaquifersandground-watersourcesofdrink-ingwaterintheUnitedStates(Zogorskiandothers,2006).

Theamountofacademic,government,andpopularlitera-tureaddressingvolatileorganiccompoundsingroundwaterisvastandscatteredamongpaperandelectronicvenues,somepublishedandsomeunpublished.Reviewingthisliteraturewouldbeadauntingtaskandcertainlybeyondthescopeforthisreport.Fortunately,anumberofpublishedpapersandreportsarereadilyavailablethatreviewed,compiled,orsummarizedtheproperties,chemistry,ordegradationpathsofVOCsingroundwater.Citationsforseveralofthesecompilationsandtheinformationsummarizedarelistedintable1.Unlessareportprovidednewlysynthesizedinformation,allfactsorinterpreta-tionsdescribedinsummaryreportsorliteraturereviewsarecitedusingtheprimarysourcereportedinthepublication.

Table 1. List of selected publications providing literature reviews and summaries of volatile organic compound degradation and behavior in ground water.

[BTEX,benzene,toluene,ethylbenzene,xylenes;PCE,tetrachloroethylene;VOC,volatileorganiccompound;MTBE,methyl tert-butylether]

Publication citation Subject

Aronsonandothers,1999 AerobicbiodegradationratesforBTEX,PCE

AronsonandHoward,1997 AnaerobicbiodegradationratesforBTEX,naphthalene,styrene,chlorinatedaliphaticcompounds

Azadpour-Keeleyandothers,1999 MicrobialdegradationandnaturalattenuationofVOCsingroundwater

Beek,2001 NaturaldegradationprocessesandratesforVOCs

Christensenandothers,2000 Oxidation-reductionconditionsinground-watercontaminantplumes

Howardandothers,1991 Environmentaldegradationratesofchemicalcompounds

Vogelandothers,1987 ChemicalreactionsinvolvedinVOCdegradation

Vogel,1994 Biodegradationofchlorinatedsolvents

Washington,1995 HydrolysisratesofdissolvedVOCs

Wiedemeierandothers,1998 NaturalattenuationofVOCsingroundwater

Wilsonandothers,2005 NaturalattenuationofMTBEingroundwater

AlthoughalargeamountofthecitationsintheacademicorgovernmentliteratureorontheInternetarepublishedthroughreliableagenciesorentities,somecitationsreferenceobscuresourcesorsourcesthataregenerallyinaccessibletothepublic.Becauseofthisissue,theliteraturecitedinthisreportisconfinedtothebodyofworkthatisavailableandeasilyaccessedthroughmainstreamacademicjournals,StateorFederalagenciesusingvariouslibraries,oronlinedatabasesontheInternet.

Ingeneral,theacademicliteraturefocusesonVOCsfromtwoperspectives:(1)analyticalandphysicalchemistryand(2)environmentaloccurrence,transport,andfate.Theanalyti-calandphysicalchemistryliteratureprovideinformationonthephysico-chemicalpropertiesofVOCs—suchasexperi-mentalandcomputedHenry’sLawconstants,fugacity,watersolubility,organiccarbonsolubility,octanol-waterpartitioncoefficients,partitioningamongvariousphysicalphases(thatis,gas,liquid,solid),experimentallyderivedandcomputer-simulatedreactionrates,microbialdegradation,andreactiontypes(thatis,hydrolysis,oxidation-reduction,dehalogenation).Theliteraturedescribingtheenvironmentaloccurrence,trans-port,andfateofVOCsingroundwaterprimarilydealswithsite-specificcontamination,andtheabioticandmicrobialtrans-formation,attenuation,anddegradationobservedingroundwater.Someofthosedocumentsattempttoconfirmorapplyin vitro(laboratorymicrocosm)resultstocontaminatedareasinsituandmanyarewrittenfromaremediationperspective.

Thelocal,State,territorial,andU.S.Governmentlit-eratureonVOCsingroundwatertypicallyencompassissuesimportanttoitscitizenryinanenvironmentalorregulatorycontext.Thisliteraturecommonlyinvolveslargergeographi-calareasthanthoseofatypicalacademicpaper.Withsomeexceptions,publicationsoftheU.S.GeologicalSurvey(USGS)arelessattentivetosite-specificcontaminationingroundwaterandmoreattentivetocontaminationissuesofareal,regional,ornationalimportance(Grady,2003;Hamlinandothers,2002,2005;Moran,2006;Zogorskiandothers,

2006).OneexceptionistheUSGSToxicSubstancesHydrol-ogyProgram(http://toxics.usgs.gov/),whichroutinelypub-lishesUSGSreportsandscientificarticlesinrefereedjournals.Theprimaryfocusofthatprogramissite-specificfateandtransportstudiesinvolvingtracemetalandorganic(includingVOCs)contaminationingroundwater.

IncontrasttoUSGSreports,theliteratureproducedbytheU.S.EnvironmentalProtectionAgency(USEPA)primarilyfocusesonapplyingscientificresultstoregulatoryandreme-diationissuesincompliancewiththeCleanAirandCleanWaterActsandtheiramendments,andthosestatutesunder-writingtheResourceConservationandRecoveryAct(RCRA)andtheComprehensiveEnvironmentalResponse,Compensa-tion,andLiabilityAct(CERCLA).TheOfficeofResearchandDevelopment(ORD)isthescientificresearcharmoftheUSEPAthatroutinelypublishesreportsaddressingthefateandtransportofVOCsingroundwaterwithinaregulatorycontext.Moreover,eachStateandterritorywithintheUnitedStatespublishesscientificandregulatoryliteratureregardingtheoccurrenceofVOCsingroundwateranditstransport,fate,andimpactonhumanandecologicalhealththatarespecifictothoseStates.OthergovernmentagenciessuchastheCentersforDiseaseControl(CDC)andtheATSDRpublishprintedandelectronicliteraturerelatingthepotentialhuman-healtheffectsofVOCsindrinkingwater.

Paperandelectronicliteraturepublishedbythepopularpress,suchassportsandoutdoormagazines,andpublica-tionsofenvironmentalgroupssuchastheSierraClubandtheNatureConservancytypicallyuseacademicandgovernmentpublicationsassourcesfortheirarticles.Thesearticlesareintendedtoeducatetheirreadersandmembersonenvironmen-talcontaminationandregulatoryissues.Withtheexceptionofliteraturepublishedbythepopularpress,theliteratureselectedandusedinthisreportspansthevenuesdescribedinprecedingparagraphs.Mostofthisliterature,particularlytheenviron-mentalfateandtransportliterature,focusesonfewerthan30VOCsingroundwater.

� Description, Properties, and Degradation of Selected VOCs Detected in Ground Water

http://toxics.usgs.gov/

Naming Conventions and Descriptions of Volatile Organic Compounds in Ground Water

Thecompoundsaddressedinthisreportbelongtotheclassoforganicchemicalscalledvolatileorganiccompounds(VOCs).Dependingonthesource,aVOChastwodefini-tions—onewithinaphysico-chemicalcontextandtheotherwithinaregulatorycontext.Thephysico-chemicaldefinitionofaVOCasstatedbyAustralia’sNationalPollutantInventoryis:Any chemical compound based on carbon chains or rings (and also containing hydrogen) with a vapor pressure greater than 2 mm of mercury (mm Hg) at 25 degrees Celsius (°C). These compounds may contain oxygen, nitrogen and other ele-ments. Substances that are specifically excluded are: carbon dioxide, carbon monoxide, carbonic acid, carbonate salts, metallic carbides and methane (AustralianDepartmentofEnvironmentandHeritage,2003).Aphysico-chemicaldefini-tionofVOCasexplicitasthatfromAustraliaandoriginatingintheUnitedStateswasnotfoundduringextensiveInternetsearches.IntheUnitedStates,theregulatorydefinitionofVOCisprovidedbytheUSEPAundertheCleanAirActandpublishedintheCodeofFederalRegulations—Volatile organic compound (VOC) means any compound of carbon, excluding carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, and ammonium carbonate, which participates in atmospheric photochemical reactions(U.S.EnvironmentalProtectionAgency,2000a).

Amongtheacademic,government,andpopularliterature,itiscommontofindaconfusingvarietyofnamesusedtoidentifyVOCs.Forexample,tetrachloroethene(IUPACname)isalsoknownasperchloroethylene,PCE,andtetrachloroeth-ylene(table2).Furthermore,somecompoundsareidentifiedusingthevariousbrandnamesunderwhichtheyaresold.Thenameusedtoidentifyanyparticularcompoundmaydependonanumberofvariables.Thesevariablesincludetheareaorregionwherethecompoundisused(forexample,Europe,UnitedStates,NortheasternUnitedStates,andsoforth),thetypeofpublicationreferringtothecompound(journalarticle,administrativereport,governmentreport,orpopularmaga-zine),thepopularityofthatnameinrecentlypublishedlit-erature,andtheprofessionofthepersonusingthename(thatis,analyticalchemist,environmentalscientist,environmentaltoxicologist,biologistorecologist,organicchemist,journalist,farmer,andsoforth).Becauseofthenumerousnamevaria-tionsforVOCs,attemptstomergeinformationfromavarietyofvenues“on-the-fly”foraparticularcompoundaretedious,confusing,andfraughtwitherror.Asearlyasthelate1800s,chemistsandothersrecognizedtheneedforaconsistentnam-ingconventionforallchemicalcompounds.

In1889,aninternationalconsortiumofchemists,encouragedbytheneedforaconsistentnamingconventionforallchemicalcompounds,formedtheInternationalUnionofChemistry(IUC).During1892atameetinginGeneva,

Switzerland,theIUCwasformalizedwithagoaltocreateasystemofrulesfornamingchemicalcompounds(GenevaRules).TheGenevaRulesestablishedthefoundation,theframework,andtheinitialrulesforaconsistent,internationalnamingconventionforallchemicalcompounds,includingthecomplexorganiccompounds.Sincethisfirstmeeting,theIUChasevolvedintotheInternationalUnionofPureandAppliedChemistry(IUPAC),anorganizationresponsibleforcreatingnewrulesandkeepingestablishedrulescurrent(BrownandLeMay,1977,p.723).ThehistoryoftheIUPACorganiza-tionandtheGenevaRulesestablishprovenancefortheformalnamesgiventotheVOCsdescribedinthisreport.

UndertheIUPACnamingconvention,VOCsarecom-monlyassignedtotwogeneralgroups:(1)aliphatichydro-carbons(alkanes,alkenes),and(2)aromatichydrocarbons(BrownandLeMay,1977).Analkaneisastraightchainorcyclic(ring-like;suchascycloalkane)structurethatconsistsofcarbon-carbonandcarbon-hydrogensinglebonds.Achlori-natedalkanealsocontainsatleastonechlorine-carbonsinglebond.Achemicalbondistheelectricalattractionbetweentwoatoms,onethathasanegativechargeandtheotherapositivecharge.Inorganiccompounds,thesechemicalbondsarecova-lent,meaningthattwobondedatomsshareelectrons(BrownandLeMay,1977).Analkeneistypicallyastraight-chainstructurethatcontainsatleastonecarbon-carbondoublebond.Achlorinatedalkenealsocontainsatleastonechlorine-carbonsinglebond.Thesedoublebondsindicatestrongercovalentbondsbetweentwocarbonatomsandimpartmorestabilitytothecompoundthanthesinglebondinanalkanecompound.

Incontrasttothealiphaticcompounds,aromaticcom-poundsarethosewithalternatingcarbon-carbonsingleanddoublebondsarrangedinaringstructure.Benzeneisthemostcommonlyrecognizedaromaticcompound(BrownandLeMay,1977).Chlorinatedaromaticcompoundsalsocontainonechlorine-carbonsinglebond(forexample,chlorobenzene).Aromaticcompoundsaretypicallymoreresistanttodegrada-tion(morestable)thanthealkaneandalkenecompounds.

Thealiphaticandthearomatichydrocarbonsarecom-monlysubgroupedevenfurtherbasedonthepresenceofattachedhalogenatoms(chlorineaschloro,bromineasbromo,orfluorineasfluoro)orfunctionalgroupsincluding,butnotlimitedto,alkylradicals.TheVOCsubgroupsincludethealkylbenzenes(suchasmethylbenzene),chlorinatedalkanes(suchas1,2-dichloroethane),chlorinatedalkenes(suchas1,1-dichlo-roethene),andthechlorinatedaromatics(suchas1,2-dichlo-robenzene;table2).Thealkylradicalsarethelowermolecularweightalkanesminusonehydrogenatom(table3)andarehighlyreactivecompoundsthatcaneasilydisplaceahydrogenatomonanothermolecule.Halogenatedoralkylatedaromaticssuchaschlorobenzeneortoluenearemoreeasilydegradedthanbenzeneinaerobicandanaerobicgroundwaterbecausethestabilityofthebenzeneringisreducedandtheringisweak-ened(Bordenandothers,1997).Addinghalidesoralkylgroupstotheringstructuredispersestheelectricalchargesfromthecarbon-carbonbondsontheringandweakensthatbond.

Naming Conventions and Descriptions of Volatile Organic Compounds in Ground Water �

Table �. Names and synonyms of volatile organic compounds commonly detected in ground water.[IUPAC,InternationalUnionofPureandAppliedChemistry;CAS,ChemicalAbstractServices;—,notapplicable]

IUPAC name 1 Common or alternative name (synonyms)� Other possible names� Predominant source

CAS number1

Alkyl benzenes1,2-dimethylbenzene o-xylene TheXinBTEX,dimethyltoluene,Xylol gasoline 95-47-61,3-dimethylbenzene m-xylene 108-38-31,4-dimethylbenzene p-xylene 106-42-3

ethylbenzene — TheEinBTEX,Ethylbenzol,phenyl-ethane

gasoline 100-41-4

methylbenzene toluene TheTinBTEX,phenylmethane,Methacide,Toluol,Antisal1A

gasoline 108-88-3

1,2,4-trimethylbenzene pseudocumene pseudocumol,asymmetricaltrimethyl-benzene

gasoline 95-63-6

Aromatic hydrocarbonsbenzene — TheBinBTEX,coalnaphtha,1,3,5-cy-

clohexatriene,mineralnaphthagasoline 71-43-2

naphthalene naphthene — gasoline,organicsyn-thesis

91-20-3

stryrene vinylbenzene phenethylene gasoline,organicsyn-thesis

100-42-5

Ethers2-methoxy-2-

methylpropanemethyltert-butyl

ether,MTBEtert-butylmethylether fueloxygenate 1634-04-4

Chlorinated alkaneschloroethane ethylchloride,

monochloroethanehydrochloricether,muriaticether solvent 75-00-3

chloromethane methylchloride — solvent 74-87-3

1,1-dichloroethane ethylidenedichloride — solvent,degreaser 75-34-3

1,2-dichloroethane ethylidenedichloride glycoldichloride,Dutchoil solvent,degreaser 107-06-2

tetrachloromethane carbontetrachloride perchloromethane,methanetetrachloride solvent 56-23-5

1,1,1-trichloroethane methylchloroform — solvent,degreaser 71-55-6

Chlorinated alkeneschloroethene vinylchloride chloroethylene,monochloroethene,

monovinylchloride(MVC)organicsynthesis,

degradationproduct75-01-4

1,1-dichloroethene 1,1-dichloroethylene,DCE vinylidenechloride organicsynthesis,degradationproduct

75-35-4

cis-1,2-dichloroethene cis-1,2-dichloroethylene 1,2DCE,Z-1,2-dichloroethene solvent,degradationproduct

156-59-2

trans-1,2-dichloroethene trans-1,2-dichloroethylene 1,2DCE,E-1,2-dichloroethene solvent,degradationproduct

156-60-2

dichloromethane methylenechloride — solvent 74-09-2

Chlorinated alkenestetrachloroethene perchloroethylene,PCE,

1,1,2,2-tetrachloroethyleneethylenetetrachloride,carbondichloride,

PERC®,PERK®solvents,degreasers 127-18-4

1,1,2-trichloroethene 1,1,2-trichloroethylene,TCE acetylenetrichloroethylene solvents,degreasersorganicsynthesis

79-01-6

Chlorinated aromatics

chlorobenzene monochlorobenzene benzenechloride,phenylchloride solvent,degreaser 108-90-7

1,2-dichlorobenzene o-dichlorobenzene orthodichlorobenzol organicsynthesis 95-50-1

1,2,3-trichlorobenzene 1,2,6-trichlorobenzene — organicsynthesis 87-61-6

1,2,4-trichlorobenzene 1,2,4-trichlorobenzol — organicsynthesis 102-82-11InternationalUnionofPureandAppliedChemistry,20062U.S.EnvironmentalProtectionAgency,1995


ThesimplestIUPACrulesfornamingorganicchemi-calsarethoseforthealkanecompounds.First,thecompoundisnamedforthelongestcarbon-carbon(C-C)chaininthecompound.Ifthecompoundcontainsahalogenorabranchingalkylgroup,theneachcarboninthecompoundisnumberedstartingattheendthatisclosesttothehalogenoralkylgroup.Thelongestcarbonchainmayincludetheoriginalbranchingalkylgroupandresultinabranchingalkylgroupwithadiffer-entcarbonpositionandname.AfteridentifyingthelongestC-Cchaininthecompound,thenumericalpositionofthealkylgrouporhalogen,ifany,onaparticularcarbonisdetermined.Thealkylgroupisnamedbasedonthenum-berofcarbonatomsitcontainscorrespondingtoanameinthealkaneseries(table3).ToillustratetheIUPACnamingprocess,considerthefollowingexample:beforetheIUPACrules,aseven-carbonalkane(septane)withanethylfunctionalgroup(table3)onthesecondcarbonwouldbenamed2-ethylseptane(fig.1).UnderIUPACrules,however,thelongestcarbonchaininthisexamplebeginsatthefirstcarbonintheethylgroup.Thisnewcarbonchaincontainseightcarbonatomsratherthantheoriginalsevenandisnowcalledoctane.Moreover,theoctanecompoundnowhasamethylgroupbranchingfromitsthirdcarbon.Therefore,thenewIUPACnameis3-methyloctane(fig.1).

Table �. The first four members of the straight-chain alkane series and associated alkyl radical.1

[IUPAC,InternationalUnionofPhysicalandAppliedChemistry;C,carbonatom;H,hydrogenatom;n,normal;t,tertiary;°C,degreesCelsius]

Alkane series condensed formula (IUPAC name) �

Number of carbon atoms in chain

Alkyl group, condensed formula (IUPAC name) �

Boiling point (°C)

CH4(methane) 1 CH

3--[methyl] –161

CH3CH

3(ethane) 2 CH

3CH

2--[ethyl] –89

CH3CH

2CH

3(propane) 3 CH

3CH

2CH

2--[n-propyl]

--CHCH

3CH

3[isopropyl]

–44

CH3CH

2CH

2CH

3(butane) 4 CH

3CH

2CH

2CH

2--[n-butyl]

CH3

CH3C--[t-butyl]

CH3

–0.5

UnderIUPACnamingconventions,highermolecularweightalkanesarenottypicallypresentasalkylradicalsinvolatileorganiccompounds

1BrownandLeMay,1977,tables24.1,24.22InternationalUnionofPureandAppliedChemistry,2006

HH

HCCH H

1 2 3 4 5 6 7

2-ethyl septane

ethylgroup

H

C C3 4 5 6 7

3-methyl octane

methyl group

8

C C C C CC CH C C C C C

1

2

HH

HCCH H

GENERIC IUPAC

Figure 1. Generic and International Union of Pure and Applied Chemistry (IUPAC) naming conventions for the same aliphatic compound.

Naming Conventions and Descriptions of Volatile Organic Compounds in Ground Water �

Sources of Volatile Organic Compounds Detected in Ground Water

ArelativelylargeamountofliteratureexiststhatdescribesVOCsingroundwateratspecific,knownareasofcontamination.Fewdocuments,however,describeVOCcontaminationinaregionalornationalcontext.OnereportbyArnethandothers(1989)liststhetop15VOCsdetectedingroundwaternearlandfillsintheUnitedStatesandinGer-many(table4).ThislistshowsthattheVOCscontaminatinggroundwaternearlandfillsaresimilarinbothcountries.MostoftheseVOCsarechlorinatedsolvents(CVOCs)andgasolinecompounds(gVOCs).Furthermore,thefrequencyofVOCsdetectedinrepresentativestudiescompletedonnational,regional,andsite-specificscalesintheUnitedStatesshowaremarkablesimilaritytothoseintable4(table5;DelzerandIvahnenko,2003;Moran,2006;Zogorskiandothers,2006).AlthoughthenumberofVOCsanalyzedinground-watersam-plesislargefornationalandregionalstudies,themostcom-monlydetectedcompounds,primarilyCVOCsandgVOCs,aresimilartothoseatsite-specificstudiescompletedatU.S.DepartmentofDefenseinstallations(table6).The10mostcommonlydetectedVOCsinthestudiessummarizedintables5and6aremethyltert-butylether(MTBE),tetrachlo-roethene(PCE),1,1,2-trichloroethene(TCE),methylbenzene

(toluene),1,1,1-trichloroethane(111-TCA),benzene,cis-1,2-dichloroethene(12-cDCE),1,1-dichloroethane(11-DCA),trans-1,2-dichloroethene(12-tDCE),thedimethyl-benzenes(m-,o-,p-xylenes)

Sources of Chlorinated AlkanesThechlorinatedsolventswithinthealkanegroupare

listedintable2.TheCVOCsaretypicallyusedinthemanu-facturingofindustrial,chemical,electronic,andconsumergoods(Smithandothers,1988;U.S.EnvironmentalProtectionAgency,2005b).Inaddition,thesecompoundsareheavilyusedassolventsincleaninganddegreasingproducts.Forexample,111-TCAisusedasasolventforadhesivesandinmetaldegreasing,pesticides,textileprocessing,cuttingfluids,aerosols,lubricants,cuttingoilformulations,draincleaners,shoepolishes,spotcleaners,printinginks,andstainrepellents.

Carbontetrachloride(CTET)wasusedasfeedstockfortheproductionofchlorofluorocarbongases,suchasdichlorodi-fluoromethane(F-12)andtrichlorofluoromethane(F-11),whichwereusedasaerosolpropellantsinthe1950sand1960s(Hol-brook,1992).During1974,theU.S.FoodandDrugAdminis-tration(FDA)bannedthesaleofCTETinanyproductusedinthehomeandtheUSEPAregulatedtheuseofchlorofluorocar-bongasesasaerosolsorpropellants.By2000,CTETproductionfornonfeedstockpurposeswasphased-outcompletely.

Table �. Volatile organic compounds ranked by those frequently detected in ground water near landfills and hazardous waste dumps in the United States and the Federal Republic of Germany.1

[IUPAC,InternationalUnionofPhysicalandAppliedChemistry;—,notapplicable]

Rank

United States of America Federal Republic of Germany

IUPAC name � Common or alternative name IUPAC name � Common or alternative name

1 1,1,2-trichloroethene 1,1,2-trichloroethylene,TCE tetrachloroethene perchloroethylene,tetrachloro-ethylene,PCE

2 tetrachloroethene perchloroethylene,tetrachloroethylene,PCE

1,1,2-trichloroethene 1,1,2-trichloroethylene,TCE

3 cis-1,2-dichloroethene cis-1,2-DCE trans-1,2-dichloroethene trans-1,2-DCE

4 benzene benzene trichloromethane —

5 chloroethene vinylchloride 1,1-dichloroethene 1,1-dichloroethylene,DCE

6 trichloromethane — dichloromethane methylenechloride

7 1,1,1-trichloroethane methylchloroform 1,1,1-trichloroethane methylchloroform

8 dimethylbenzene xylene 1,1-dichloroethane ethylenedichloride

9 trans-1,2-dichloroethene trans-1,2-dichloroethylene 1,2-dichloroethane ethylenedichloride

10 methylbenzene toluene phenol —

11 ethylbenzene ethylbenzene acetone dimethylketone,2-propanone,andbeta-ketopropane

12 dichloromethane methylenechloride toluene methylbenzene

13 dichlorobenzene,total — bis-(2-ethylhexyl)-phthalate —

14 chlorobenzene chlorobenzene benzene benzene

15 tetrachloromethane carbontetrachloride chloroethene vinylchloride

1Arnethandothers,1989,p.3992InternationalUnionofPureandAppliedChemistry,2006


Table �. Volatile organic compounds detected in regional and national ground-water studies in the United States.[µg/L,microgramsperliter;[12],percentageofsamplesabovetheanalyticalreportinglimit;<,lessthan;ND,notdetectedaboveanalyticalreportinglevel]

RankStatewide, ground water

in Wisconsin1

Ground water in the Santa Ana River Basin, California�

Ground-water and drinking-water supply wells in the United States (concentrations greater than 0.� µg/L)�

Aquifer studies� Domestic water-supply wells� Public water-supply wells�

1 dichloromethane[16.3] 1,1,2-trichloroethene(TCE)[12] tetrachloroethene(PCE)[3.7] 2-methoxy-2-methylpropane(MTBE)[2.9]

2-methoxy-2-methylpropane(MTBE)[5.4]

2 1,1-dichloroethane[13.6] 1,1,1-trichloroethane[10.5] 2-methoxy-2-methylpropane(MTBE)[2.8]

tetrachloroethene(PCE)[2.0] tetrachloroethene(PCE)[5.3]

3 cis-1,2-dichloroethene,1,1-dichloro-ethane[13.6]

tetrachloroethene(PCE)[9.1] 1,1,2-trichloroethene(TCE)[2.6] 1,1,1-trichloroethane[1.4] 1,1,2-trichloroethene(TCE)[4.3]

4 1,1,2-trichloroethene(TCE)[13.3] 1,1-dichloroethene[5.7] methylbenzene[1.9] methylbenzene[1.0] 1,1,1-trichloroethane[2.2]

5 methylbenzene[11.6] 2-methoxy-2-methylpropane(MTBE)[5.3]

1,1,1-trichloroethane[1.7] chloromethane[.97] 1,1-dichloroethane[2.0]

6 tetrachloroethene(PCE)[9.8] cis-1,2-dichloroethene[4.3] chloromethane[1.1] 1,1,2-trichloroethene(TCE)[.92] cis-1,2-dichloroethene[1.5]

7 benzene[8.5] methylbenzene[3.8] trans-1,2-dichloroethene[0.91] dichloromethane[.67] 1,1-dichloroethene(DCE)[1.3]

8 chloroethene[8.0] 1,1-dichloroethane[2.9] dichloromethane[0.89] 1,2,4-trimethylbenzene[.32] trans-1,2-dichloroethene[1.0]

9 1,3-and1,4-dimethylbenzenes[7.9] benzene[1.4] 1,1-dichloroethane[0.86] 1,1-dichloroethane[.29] methylbenzene[1.0]

10 1,1,1-trichloroethane[7.8] 1,2-dimethylbenzene[1.4] 1,1-dichloroethene[0.66] benzene,1,2-dichloroethane[.21] tetrachloromethane[.73]

11 ethylbenzene[7.6] 1,3-and1,4-dimethylbenzene[1.4] benzene[.63] tetrachloromethane[.21] 1,3-and1,4-dimethylbenzene[0.60]

12 1,2,4-trimethylbenzene[7.1] trans-1,2-dichloroethene[<1] 1,2,4-trimethylbenzene[.63] 1,1-dichloroethene[.21] 1,2-dichloroethane[.56]

13 1,2-dimethylbenzene[6.8] dichloromethane[<1] 1,2-dichloroethane[.47] totalxylenes[0.21] 1,2-dimethylbenzene[.48]

14 chloromethane[6.7] ethylbenzene[<1] cis-1,2-dichloroethene[.42] cis-1,2-dichloroethene[.18] benzene,dichloromethane,ethylbenzene[.46]

15 naphthalene[6.5] naphthalene[<1] totalxylenes[.38] naphthalene[.15] chloromethane[.38]

16 chloroethane[6.3] tetrachloromethane[<1] tetrachloromethane[.31] ethylbenzene[.12] 1,2,4-trimethylbenzene[0.32]

17 chlorobenzene[4.3] 1,2,4-trimethylbenzene[<1] chloroethane[.29] chloroethane[.093] chloroethane[.28]

18 1,2-dichloroethane[3.7] chlorobenzene[ND] chloroethene[.26] chloroethene[.083] vinylbenzene[.19]

19 trans-1,2-dichloroethene[3.3] chloroethane[ND] ethylbenzene[.26] trans-1,2-dichloroethene[.045] chlorobenzene,1,2-dichlorobenzene[.18]

20 1,1-dichloroethene(DCE)[2.6] chloromethane[ND] chlorobenzene[.17] chlorobenzene[.042] chloroethene[.18]

21 1,2-dichlorobenzene[2.4] chloroethene[ND] naphthalene[.16] 1,2-dichlorobenzene[.042] naphthalene[.10]

22 2-methoxy-2-methylpropane(MTBE)[2.3]

1,2-dichlorobenzene[ND] 1,2-dichlorobenzene[.12] vinylbenzene[ND] 1,2,4-trichlorobenzene[ND]

23 tetrachloromethane[1.8] 1,2-dichloroethane[ND] vinylbenzene[ND] 1,2,4-trichlorobenzene[ND] 1,3-dichlorobenzene[ND]

24 vinylbenzene[1.2] 1,2,3-trichlorobenzene[ND] 1,2,3-trichlorobenzene[ND] 1,1,2-trichloroethane[ND] 1,1,2-trichloroethane[ND]

25 1,2,4-trichlorobenzene,1,1,2-trichloroethane[<1.0]

vinylbenzene[ND] 1,2,3-trichlorobenzene[ND] 1,2,3-trichlorobenzene[ND]

11,305–4,086samples(WisconsinDepartmentofNaturalResources,2000)29–112samples(Hamlinandothers,2002)3Zogorskiandothers,200641,710–3,498samples51,190–1,208samples6828–1,096samples

Sources of Volatile Organic Compounds Detected in Ground W

ater

�

Chemicalmanufacturingisthelargestuseof11-DCAand1,2-dichloroethane(12-DCA).Bothcompoundsserveasanintermediateduringthemanufactureofchloroethene(vinylchloride,VC),111-TCA,andtoalesserextenthigh-vacuumrubber.BothDCAisomersalsoareusedasasolventforplastics,oils,andfats,andincleaningagentsanddegreasers(AgencyforToxicSubstancesandDiseaseRegistry,1990c,p.51;2001,p.160).About98percentofthe12-DCApro-ducedintheUnitedStatesisusedtomanufactureVC.Smalleramountsof12-DCAareusedinthesynthesisofvinylidenechloride,TCE,PCE,aziridines,andethylenediamines,andinotherchlorinatedsolvents(U.S.EnvironmentalProtectionAgency,1995).

Thecompound111-TCAwasinitiallydevelopedasasafersolventtoreplaceotherchlorinatedandflammablesolvents.Thecompoundisusedasasolventforadhesives(includingfoodpackagingadhesives)andinmetaldegreasing,pesticides,textileprocessing,cuttingfluids,aerosols,lubricants,cuttingoilformulations,draincleaners,shoepolishes,spotcleaners,print-

inginks,andstainrepellents,amongotheruses(AgencyforToxicSubstancesandDiseaseRegistry,2004,p.181).TheotherTCAisomer,1,1,2-trichloroethane(112-TCA),haslimiteduseasacommon,general-usesolventbutisusedintheproductionofchlorinatedrubbers(Archer,1979).Insomecases,112-TCAmaybesoldforuseinconsumerproducts(AgencyforToxicSubstancesandDiseaseRegistry,1989,p.59).

Before1979,thesinglelargestuseofchloroethanewasintheproductionoftetraethyllead.Asrecentlyas1984,thedomesticproductionoftetraethylleadaccountedforabout80percentofthechloroethaneconsumedintheUnitedStates;whereasabout20percentwasusedtoproduceethylcellulose,andusedinsolvents,refrigerants,topicalanesthetics,andinthemanufactureofdyes,chemicals,andpharmaceuticals.Sincethe1979banontetraethylleadingasolineanditssub-sequentphaseoutinthemid-1980,theproductionofchloro-ethaneinrecentyearshasdeclinedsubstantiallyintheUnitedStates(AgencyforToxicSubstancesandDiseaseRegistry,1998,p.95).

Table �. Volatile organic compounds detected in ground-water case studies at selected U.S. Department of Defense installations.

[[43.3],percentageofsampleswithadetectedconcentration;ND,notdetectedaboveanalyticalreportinglevel]

RankDover Air Force Base, Maryland 1

U.S. Army Armament Research and Development Center,

Picatinny, New Jersey, 19�8 – 8� �

U.S. Naval Undersea Warfare Center,

Washington, D.C. �

Wright-Patterson Air Force Base, Ohio,

199� – 9� �

1 2-methoxy-2-methylpropane(MTBE)[25.5]

1,1,2-trichloroethene(TCE)[58.5]

chloroethene[64] 1,1,2-trichloroethene(TCE)[12.5]

2 cis-1,2-dichloroethene[21.7] tetrachloroethene(PCE)[24.9] cis-1,2-dichloroethene[59] tetrachloroethene(PCE)[5.8]

3 1,1,2-trichloroethene(TCE)[20.3] trans-1,2-dichloroethene(DCE)[18.6]

trans-1,2-dichloroethene[44.8]

1,1,1-trichloroethane[2.3]

4 tetrachloroethene(PCE)[13.7] 1,1,1-trichloroethane[16.8] 1,1,2-trichloroethene(TCE)[40.4]

chloromethane[2.3]

5 benzene[10.4] 1,1-dichloroethane[9.6] totalBTEXcompounds[40.1] cis-andtrans-1,2-dichloroethene[1.2]

6 methylbenzene[6.6] cis-1,2-dichloroethene(DCE)[9.6]

1,1-dichloroethane[37.2] chloroethene[.9]

7 dimethylbenzenes(m-,p-xylene)[3.7]

methylbenzene(toluene)[4.4] chloroethane[33.9] dichloromethane[.9]

8 ethylbenzene[2.3] benzene[2.6] 1,1-dichloroethene[31.3] methylbenzene[.6]

9 chloroethene[ND] — tetrachloroethene(PCE)[9.6] benzene[.3]

10 — — 1,1,1-trichloroethane[6.9] chloroethane[.3]

11 — — — tetrachloromethane[.3]1212samples(BarbaroandNeupane,2001;Guertalandothers,2004)

2607samples(Sargentandothers,1986)

3121–179samples(Dinicolaandothers,2002)

4343samples(Schalkandothers,1996)

8 Description, Properties, and Degradation of Selected VOCs Detected in Ground Water

Sources of Chlorinated Alkenes and Benzenes

Thechlorinatedalkeneslistedintable2includetwoofthemostwidelyusedanddistributedsolventsintheUnitedStatesandEurope.Thesesolvents,PCEandTCE,alsoareamongthemostcommoncontaminantsingroundwater(tables5and6).ThetextileindustryusesthelargestamountofPCEduringtheprocessing,finishingofrawandfinishedtextiles,andforindustrialandconsumerdrycleaning(U.S.EnvironmentalProtectionAgency,2005b,Webpage:http://www.epa.gov/opptintr/chemfact/f_perchl.txt,accessedMay23,2006).MostoftheTCEusedintheUnitedStatesisforvapordegreasingofmetalpartsandsometextiles(U.S.EnvironmentalProtectionAgency,2005b,Webpage:http://www.epa.gov/OGWDW/dwh/t-voc/trichlor.html,accessedMay23,2006).OtherusesofPCEandTCEincludemanufacturingofpharmaceuticals,otherorganiccompounds,andelectroniccomponents,andinpaintandinkformulations(Smithandothers,1988).

Fourchlorinatedbenzenescommonlydetectedinground-watercontaminationstudiesincludechlorobenzene(CB),1,2-dichlorobenzene(12-DCB),andtwoisomersoftrichlo-robenzene,1,2,3-trichlorobenzene(123-TCB)and1,2,4-tri-chlorobenzene(124-TCB;tables5and6).Chlorobenzeneiscommonlyusedasasolventforpesticideformulations,inthemanufacturingofdi-isocyanate,asadegreaserforauto-mobileparts,andintheproductionofnitrochlorobenzene.Solventusesaccountedforabout37percentofchlorobenzeneconsumptionintheUnitedStatesduring1981(AgencyforToxicSubstancesandDiseaseRegistry,1990a,p.45).Thecompound12-DCBisusedprimarilytoproduce3,4-dichloro-anilineherbicides(AgencyforToxicSubstancesandDiseaseRegistry,1990b,p.263).Thetwotrichlorobenzeneisomersareprimarilyusedasdyecarriersinthetextileindustry.Otherusesincludeseptictankanddraincleaners,theproductionofherbicidesandhigherchlorinatedbenzenes,aswoodpreserva-tives,andinheat-transferliquids(U.S.EnvironmentalProtec-tionAgency,2005b,Webpage:http://www.epa.gov/OGWDW/dwh/t-voc/t-124-tric.html,accessedMay23,2006).

Sources of Gasoline CompoundsAtabasiclevel,gasolineproductionissimplyaprocessof

sequentialdistillationsthatseparate,byvaporization,volatilehydrocarbonsfromcrudeoil.Typically,thesehydrocarbonsarethelowermolecularweightcompoundsthatcommonlyarethemostvolatilecompoundsincrudeoil.Moreadvancedmethodssuchasheat“cracking”areusedtobreakdownthecomplexaromatichydrocarbonsincrudeoilintosmaller,morevolatilecompoundsthatareeasilydistilled.Oncethehydrocarbonsareinavaporform,acondensationprocesscoolsthevaporandthe

resultingliquidiscollectedforfurtherrefining.Thehydro-carboncompositionofgasolinedependsonthesourceofthecrudeoilused,therefiningprocess,therefiner,theconsumerdemand,thegeographiclocationoftherefinery,andthedistri-butionalareaofthegasoline(HarperandLiccione,1995).

Gasolineistypicallyamixtureofvarioushydrocarbonsthatincludealkanes,cycloalkanes,cycloalkenes,akylben-zenes,andaromaticcompounds,andsomeoxygenatedalcoholadditives(table7).Manyofthehydrocarbonsingasolineareadditivesandblendingagentsintendedtoimprovetheperformanceandstabilityofgasoline.Theseadditivestypi-callyconsistofoxygenatessuchasmethyltert-butylether(MTBE),ethanol,ormethanol,antiknockagents,antioxidants,metaldeactivators,leadscavengers,antirustagents,anti-icingagents,upper-cylinderlubricants,detergents,anddyes.Attheendoftherefiningprocess,finishedgasolinecommonlycontainsmorethan150separatecompounds;however,someblendsmaycontainasmanyas1,000compounds(HarperandLiccione,1995).

Table �. Major organic compounds in a typical gasoline blend.1

[n,C5-C

13carbonchain;MTBE,methyltert-butylether;

TBA,tert-butylalcohol]

Major compounds Percent composition by weight

n-alkanes 17.3

Branchedalkanes 32.0Cycloalkanes 5.0Olefins 1.8Aromatichydrocarbons 30.5 Benzene 3.2 Toluene 4.8 Ethylbenzene 1.4 Xylenes 6.6 Otherbenzenes 11.8 Otheraromatics 2.7

Other possible additives

Octaneenhancers:MTBE,TBA,ethanolAntioxidants:N,N'-dialkylphenylenediamines,di-andtri-

alkylphenols,butylatedmethyl,ethylanddimethylphenolsMetaldeactivators:variousN,N'-disalicylidenecompoundsIgnitioncontrollers:tri-o-cresylphosphate(TOCP)Detergents/dispersants:alkylaminephosphates,poly-isobutene

amines,long-chainalkylphenols,alcohols,carboxylicacids,andamines

Corrosioninhibitors:phosphoricacids,sulfonicacids,carboxylicacids

1HarperandLiccione,1995

Sources of Volatile Organic Compounds Detected in Ground Water 9

http://www.epa.gov/OGWDW/dwh/t-voc/trichlor.html

http://www.epa.gov/opptintr/chemfact/f_perchl.txt

http://www.epa.gov/OGWDW/dwh/t-voc/124-tric.html

http://www.epa.gov/OGWDW/dwh/t-voc/124-tric.html

BTEX Compounds (Benzene, Toluene, Ethylbenzene, and Xylene)

About16percentofatypicalgasolineblendconsistsofBTEXcompounds(collectively,benzene,toluene,ethlyben-zene,andthreexylenecompounds;table7).Ofthedifferentcomponentscontainedingasoline,BTEXcompoundsarethelargestgroupassociatedwithhuman-healtheffects.Becauseoftheadverseimpactonhumanhealth,BTEXcompoundsaretypicallythefuelcomponentsanalyzedinground-watersamplescollectedfromfuel-contaminatedaquifers.Further-more,threeminorcomponentsofgasoline:naphthalene,vinylbenzene(styrene),and1,2,4-trimethylbenzene(124-TMB)arecommonlydetectedalongwithBTEXcompoundsandMTBEincontaminatedgroundwater(tables5and6).AlthoughtheindividualBTEXcompoundsarewidelyusedassolventsandinmanufacturing(Swoboda-Colberg,1995),gasolineleaksfromundergroundstoragetanksanddistributionpipelinesistheprimarycontributorofBTEXcontaminationingroundwater(U.S.EnvironmentalProtectionAgency,2000b;U.S.EnvironmentalProtectionAgency,2005a).

Methyl Tert-butyl EtherMethyltert-butylether(MTBE;IUPAC2-methoxy-2-

methylpropane)isagasolineadditivewithintheclassoffueloxygenates.Oxygenatesareorganiccompoundsthatenrichgasolinewithoxygentoimprovethecombustionefficiencyofgasolineandreducecarbonmonoxideemissionsinvehicleexhaust.Sincethelate1980s,gasolineshippedtoareasoftheUnitedStatesthatfallundertheReformulatedGasoline(RFG)andOxygenatedFuel(Oxyfuel)ProgramsoftheCleanAirAct(CAA)anditsamendmentshascontainedoxygenates(Moranandothers,2004).Moreover,30percentofthegaso-lineusedintheUnitedStatessince1998containedoxygen-atesincompliancewithRFGrequirementswhile4percentofthegasolineusedcompliedwiththeOxyfuelrequirements(U.S.EnvironmentalProtectionAgency,1998).Reformulatedgasolinecontainsabout11percentMTBEbyvolume(DelzerandIvahnenko,2003).

Basic Properties of Selected Volatile Organic Compounds

Volatileorganiccompoundshaveanumberofuniquepropertiesthatbothinhibitandfacilitateground-watercon-tamination.Tables8through12listbasicphysicalpropertiesof27VOCsdetectedingroundwater.Physicalpropertiesuniquetoeachcompoundtypicallyaregovernedbythenumberofcarbonsandthecovalentbondinginthecompound,thenumberandlocationofchlorineatoms,andthenumber,locationandtypeofalkylgroups.ThephysicalpropertiesaddressedinthisreportincludetheHenry’sLawconstant(H),watersolubil-ity,density,octanol-waterpartitioning(LogK

ow),andorganic

carbonpartitioning(LogKoc

)ofthenon-aqueousphaseliquid(NAPL).ModelsthatestimatethefateandtransportofVOCsingroundwaterdependontheaccuracyandreliabilityofphys-icalpropertymeasurements.Somemodels,suchasthefugacitymodels,alsousethesepropertiestopredictacompound’srateofmovementintoandoutofenvironmentalcompart-ments(soil,water,air,orbiota;Mackay,2004).Predictingtheenvironmentalfateofacompoundingroundwaterdependsondatathatquantifies:(1)thecompound’stendencytovolatilize(gaseousphase),(2)todissolveinwater(aqueousphase),(3)tofloatonorsinkbeneaththewatersurface,(4)todissolveinorsorbtootherorganiccompounds(includingnaturalorganicmatter),and(5)thecompound’saffinityforionicallychargedsurfacessuchasclayorsoilparticles.Fugacitymodelsofvaryingcomplexityareincommonuseandrelyonthephysi-calpropertiesofthesecompoundstoestimateplumemigrationandpersistence,andtoguidetheremediationofcontaminatedgroundwater(Mackayandothers,1996;InstituteforEnviron-mentalHealth,2004;SaichekandReddy,2005).

Degradation of Selected Volatile Organic Compounds in Ground Water

Underspecificconditions,mostorganiccompoundsdegradeataparticularrateduringagivenlengthoftime.Thespeedofthedegradationdependsonthepresenceandactivityofmicrobialconsortia(bacteriaandfungispecies),environmentalconditions(temperature,aquifermaterials,organicmattercon-tent),andtheavailabilityandconcentrationofcarbonsources(primarysubstrate)availabletothemicrobialconsortia.Thepri-marysubstratecanbeaVOCororganiccarbonfounddissolvedinwaterorsorbedtoaquifersediments.Whenprimarysubstrateconcentrationsaresmall,themicrobialpopulationissmallandbiodegradationratesarerelativelyslow.Asthesubstratecon-centrationsincrease,themicrobialpopulationgrowsandthedegradationrateincreasesconcomitantly(BradleyandChapelle,1998).Themicrobialpopulationwillgrowuntiltheyreachamaximumgrowthrate(Aronsonandothers,1999).

ThedegradationofVOCsingroundwaterisatransfor-mationofaparentcompoundtodifferentcompoundscom-monlycalleddaughterproducts,degradates,ordegradationby-products.Thesetransformationscanbegroupedintotwogeneralclasses:(1)thosethatrequireanexternaltransferofelectrons,calledoxidation-reductionreactions;and(2)thosethatdonotinvolveatransferofelectrons,calledsubstitutionsanddehydrohalogenations(Vogelandothers,1987).Table13summarizesthesereactions.Oxidation-reductionreactionsarethedominantmechanismsdrivingVOCdegradationandmostofthesereactionsarecatalyzedbymicroorganisms(Wiedemeierandothers,1998;Azadpour-Keeleyandothers,1999).Substitutionreactionsthatcanremovechlorineatoms,suchashydrolysis,candegradesomechlorinatedalkanes(trichloroethane)tononchlorinatedalkanes(ethane)withorwithoutamicrobialpopulationcatalyzingthereaction(Vogel


andothers,1987;Olaniranandothers,2004).Typically,thepolychlorinatedcompounds(forexample,PCEandTCE)eas-ilydegradeunderanaerobicconditionsandarelessmobileinsoilandaquifermaterialsthanthedi-andmono-chlorinatedcompounds(fig.2).Degradationpathwaysareillustratedin

figures3through19forasubsetofthecompoundslistedintable2.ThesefiguresaremodificationsofpathwaysdescribedintheUniversityofMinnesota’sbiodegradation/biocatalysisdatabase(Ellisandothers,2006)accessibleviatheInternetathttp://umbbd.msi.umn.edu,accessedMay23,2006.

Table 8. Henry’s Law constants for selected volatile organic compounds detected in ground water.

[IUPAC,InternationalUnionofPhysicalandAppliedChemistry;kPa,kilopascals;m3,cubicmeter;mol,mole;°C,degreesCelsius;—,notapplicable]

IUPAC name1 Common or alternative name� Henry’s Law� constant (H) (kPa m� mol–1 at ��°C)

tetrachloromethane carbontetrachloride 2.99

Incr

easi

ngte

nden

cyf

ora

com

poun

dto

mov

efr

omth

ew

ater

ph

ase

toth

eva

por

phas

ew

hen

ine

quili

briu

mw

ithp

ure

wat

er

chloroethene vinylchloride,chloroethylene 2.68

1,1-dichloroethene 1,1-dichloroethylene,DCE 2.62

1,1,1-trichloroethane methylchloroform 1.76

tetrachloroethene perchloroethylene,tetrachloroethylene,PCE 1.73

chloroethane ethylchloride,monochloroethane 41.11

1,1,2-trichloroethene 1,1,2-trichloroethylene,TCE 1.03

trans-1,2-dichloroethene trans-1,2-DCE,trans-1,2-dichloroethylene .960

chloromethane methylchloride 5.920

ethylbenzene — .843

1,3-dimethylbenzene m-xylene .730

1,4-dimethylbenzene p-xylene .690

methylbenzene toluene .660

1,1-dichloroethane 1,1-ethylidenedichloride .630

benzene — .557

1,2-dimethylbenzene o-xylene .551

1,2,4-trimethylbenzene pseudocumene .524

cis-1,2-dichloroethene cis-1,2-dichloroethylene,cis-1,2-DCE .460

chlorobenzene monochlorobenzene .320

stryrene vinylbenzene .286

1,2,4-trichlorobenzene 1,2,4-trichlorobenzol .277

1,2,3-trichlorobenzene 1,2,6-trichlorobenzene .242

1,2-dichlorobenzene o-dichlorobenzene .195

1,2-dichloroethane 1,2-ethylidenedichloride,glycoldichloride .140

1,1,2-trichloroethane methylchloroform .092

2-methoxy-2-methylpropane methyltert-butylether,MTBE .070

naphthalene naphthene .043

1InternationalUnionofPureandAppliedChemistry,20062U.S.EnvironmentalProtectionAgency,19953Lide,20034Gossett,19875NationalCenterforManufacturingSciences,2006

Degradation of Selected Volatile Organic Compounds in Ground Water 11

http://umbbd.msi.umn.edu

Table 9. Water-solubility data for selected volatile organic compounds detected in ground water.

[IUPAC,InternationalUnionofPureandAppliedChemistry;mg/L,milligramsperliter;°C,degreesCelsius;—,notapplicable]

IUPAC name1 Common or alternative name� Water solubility� (mg/L at ��°C)

2-methoxy-2-methylpropane methyltert-butylether,MTBE 36,200

Incr

easi

nga

mou

nto

fno

n-aq

ueou

sph

ase

liqui

dth

atc

and

isso

lve

inw

ater

1,2-dichloroethane 1,2-ethylidenedichloride,glycoldichloride 8,600

chloromethane methylchloride 45,320

chloroethane ethylchloride,monochloroethane 56,710

cis-1,2-dichloroethene cis-1,2-dichloroethylene 6,400

1,1-dichloroethane 1,1-ethylidenedichloride 5,000

1,1,2-trichloroethane methylchloroform 4,590

trans-1,2-dichloroethene trans-1,2-dichloroethylene 4,500

chloroethene vinylchloride,chloroethylene 2,700

1,1-dichloroethene 1,1-dichloroethylene,DCE 2,420

benzene — 1,780

1,1,1-trichloroethane methylchloroform 1,290

1,1,2-trichloroethene 1,1,2-trichloroethylene,TCE 1,280

tetrachloromethane carbontetrachloride 1,200

methylbenzene toluene 531

chlorobenzene — 495

stryrene vinylbenzene 321

tetrachloroethene perchloroethylene,tetrachloroethylene,PCE 210

1,2-dimethylbenzene o-xylene 207

1,4-dimethylbenzene p-xylene 181

1,3-dimethylbenzene m-xylene 161

ethylbenzene — 161

1,2-dichlorobenzene o-dichlorobenzene 147

1,2,4-trimethylbenzene pseudocumene 57

1,2,4-trichlorobenzene 1,2,4-trichlorobenzol 37.9

naphthalene naphthene 631.0

1,2,3-trichlorobenzene 1,2,6-trichlorobenzene 30.9

1InternationalUnionofPureandAppliedChemistry,2006

2U.S.EnvironmentalProtectionAgency,1995

3Lide,2003

4NationalCenterforManufacturingSciences,2006

5Horvath,1982

6Lyman,1982

1� Description, Properties, and Degradation of Selected VOCs Detected in Ground Water

Table 10. Density of selected volatile organic compounds detected in ground water compared to the density of water at 20 degrees Celsius.

[IUPAC,InternationalUnionofPureandAppliedChemistry;g/cm,gramspercentimeter;°C,degreesCelsius;—,notapplicable]

IUPAC name1 Common or alternative name� Density � (g/cm�, �0°C)


Incr

easi

ngd

ensi

ty(

heav

ier

than

wat

er)







1,2-dichlorobenzene o-dichlorobenzene 1.306

cis-1,2-dichloroethene cis-1,2-dichloroethylene 1.284

trans-1,2-dichloroethene trans-1,2-dichloroethylene 1.256

1,2-dichloroethane 1,2-ethylidenedichloride,glycoldichloride 1.235


1,1-dichloroethane 1,1-ethylidenedichloride 1.176

chlorobenzene monochlorobenzene 1.106

purewaterat20°C 1.000

naphthalene naphthene .997

Dec

reas

ing

dens

ity(

light

erth

anw

ater

)

chloromethane methylchloride .991

chloroethane ethylchloride .920


stryrene vinylbenzene .906

1,2-dimethylbenzene o-xylene .880

benzene — .876

ethylbenzene — .867

1,2,4-trimethylbenzene pseudocumene .876

methylbenzene toluene .867

1,3-dimethylbenzene m-xylene .864

1,4-dimethylbenzene p-xylene .861




3Lide,2003

4Chiouandothers,1983

Degradation of Selected Volatile Organic Compounds in Ground Water 1�

Table 11. Octanol-water partition coefficients for selected volatile organic compounds detected in ground water.

[IUPAC,InternationalUnionofPureandAppliedChemistry;Kow

,octanol-waterpartitioncoefficient;—,notapplicable]

IUPAC name1 Common or alternative name� Octanol/ water partition coefficient � (Log Kow)


In

crea

sing

aff

inity

for

org

anic

mat

ter

and

lipid

s


1,2,4-trimethylbenzene pseudocumene 3.65

1,2-dichlorobenzene o-dichlorobenzene 3.46


1,3-dimethylbenzene m-xylene 3.20

ethylbenzene ethylbenzene 3.15

1,4-dimethylbenzene p-xylene 3.15

1,2-dimethylbenzene o-xylene 3.12

stryrene vinylbenzene 3.05



methylbenzene toluene 2.73






benzene — 2.13

trans-1,2-dichloroethene trans-1,2-dichloroethylene 1.93

cis-1,2-dichloroethene cis-1,2-dichloroethylene 1.86



chloroethane ethylchloride 1.43



chloromethane methylchloride .91



3Sangster,1989

4Mackayandothers,1992a


Table 1�. Soil-sorption partition coefficients for selected volatile organic compounds detected in ground water.

[IUPAC,InternationalUnionofPureandAppliedChemistry;Koc

,soilorganiccarbonpartitioncoefficient;—,notapplicable]

IUPAC name1 Common or alternative name� Soil-sorption coefficient (Log Koc in soil)

1,2,4-trimethylbenzene pseudocumene 33.34

Incr

easi

nga

ffin

ityf

ors

oilo

rgan

icm

atte

r

1,2,3-trichlorobenzene 1,2,6-trichlorobenzene 43.18–33.42



vinylbenzene styrene 22.72–2.74

1,2-dichlorobenzene o-dichlorobenzene 62.46–52.51


ethylbenzene — 52.22


1,3-dimethylbenzene m-xylene 72.11–2.46




1,1,2-trichloroethane methylchloroform 71.78–2.03


methylbenzene toluene 71.75–102.28


1,2-,1,4-dimethylbenzene o-xylene,p-xylene 21.68–1.83

chloroethane ethylchloride 41.62

cis-1,2-dichloroethene cis-1,2-dichloroethylene 21.56–1.69


trans-1,2-dichloroethene trans-1,2-dichloroethylene 21.56–1.69


benzene — 51.49–71.73

methyltert-butylether MTBE 111.09

chloromethane methylchloride 3.778



3Boydandothers,1990

4SchwarzenbachandWestall,1981



7Seipandothers,1986

8Frieselandothers,1984

9Kileandothers,1996

10GarbariniandLion,1986


12U.S.EnvironmentalProtectionAgency,2005b


http://www.epa.gov/enviro/html/emci/chemref/75343.html

Table 1�. Common abiotic and biotic reactions involving halogenated aliphatic hydrocarbons.1

[+,plus;Cl,chloride]

Reactions Potential reaction products

Substitution

abiotichydrolysis alcoholthenanacidordiol(chloroethanolchloroaceticacid)

biotichydrolysis alcoholthenanacidordiolviamicrobialenzymes(hydrolasesorglutathioneS-transferases;(chloroethanolchloroaceticacid)

conjugationornucleophilicreactions(biotic) freehalideplusanewcompoundwiththenucleophileorconjugate

Dehydrohalogenation

dehydrohalogenation halogenatedacid(chloroaceticacid),alkanetoalkene(dichloroethanechloroethane)

Oxidation

α-hydroxylation monochlorinatedalkanetoamonochlorinatedalcohol(chloroethanechloroethanol)

halosyloxidation monohalogenatedalkanetoanonhalogenatedalkane(chloroethaneethane+Cl)

epoxidation halogenatedepoxidecompound

biohalogenation nonhalogenatedalkenetoamonohalogenatedalcohol(ethene+Clchloroethanol)

Reduction

hydrogenolysis freehalideandnonhalogenatedcompound(chloroethaneCl+ethane)

dihaloelimination dihalogenatedalkanetoanonhalogenatedalkene(dichloroethane ethene)

coupling combiningoftwohalogenatedcompoundsintoonehalogenatedcompound

1Vogelandothers,1987,figure1

Aerobic degradation

Sorption

Reductive dechlorination rate

DEGREE OF CHLORINATION

PolychlorinatedMonochlorinated

DEGR

ADAT

ION

RAT

E

Sorp

tion

onto

sub

surfa

ce m

ater

ial

0.25 4

Figure �. Relation between degree of chlorination and anaerobic reductive-dechlorination, aerobic degradation and sorption onto subsurface material (modified from Norris and others, 1993, p. 10–19). Degree of chlorination is number of chloride atoms divided by number of carbon atoms.


Aquiferconditions(aerobicandanaerobic)andmicrobialmetabolism(respiration,fermentation,andco-metabolism)controltheenvironmentaldegradationofVOCsingroundwater.Inaerobicenvironments,oxygenservesastheterminalelectronacceptor(TEA)andcompoundssuchasMTBEandBTEXaresubsequentlydegraded(oxidized)toothercompounds(Azad-pour-Keeleyandothers,1999).Furthermore,underaerobiccon-ditionsCVOCscanbeinadvertentlydegraded(co-metabolized)vianonspecificenzymes(oxygenases)producedbymicroor-ganismsduringthemetabolismofothercompoundsservingasprimarysubstrates(forexample,BTEX,methane,propane,toluene,ammonia,ethene,ethane).AlthoughtheaerobicmineralizationofmostVOCsultimatelyyieldscarbondioxideandwater,co-metabolicbiodegradationofCVOCsgenerallyproceedsviaanunstableepoxideintermediatethatspontane-ouslydecomposestocarbondioxide,chloride,orotherorganicby-productssuchasacetate(Robertsandothers,1986).

Anaerobicdegradationistypicallyaseriesofdecarbox-ylationsandoxidation-reduction(redox)reactionscatalyzedeitherbysinglemicroorganismsorbyaconsortiumofmicroorganisms(Dolfing,2000).Duringanaerobicdegra-dation,CVOCsfunctionasterminalelectronacceptorsinaprocesscalledreductivedechlorination(Vogelandoth-ers,1987).Theoretically,reductivedechlorinationisthesequentialreplacementofonechlorineatomonachlorinatedcompoundwithahydrogenatom.Thereplacementcontin-uesuntilthecompoundisfullydechlorinated.Forexample,PCEcanundergoreductivedechlorinationtoless-chlorinatedcompounds,suchasTCEor12-DCE,ortononchlorinatedcompoundssuchasethene,ethane,ormethane(methano-genesis).Eachsuccessivestepinthedechlorinationprocessistheoreticallyslowerthantheprecedingstep.Thedechlo-rinationprocessslowsbecauseaschlorinesareremovedtheenergycoststoremoveanotherchlorineatomincreases(freeenergyofthereactiondecreases;Dolfing,2000).Asaresult,biodegradationmaynotproceedtocompletioninsomeaquifersleavingintermediatecompounds(forexample,dichloroethenesandvinylchloride)toaccumulateingroundwater(Azadpour-Keeleyandothers,1999).Otherconstraintsonbiodegradationsuchasareductioninorlossofprimarysubstrate,ormicrobialsuppressionalsocanplayaroleintheaccumulationofintermediatecompounds.ThisisaparticularconcernwithVCbecauseitisaknownhumancarcinogen(AgencyforToxicSubstancesandDiseaseRegistry,2005)anditsaccumulationmaycreateahealthissuethatmightnotbeaconcernduringtheearlystagesofground-watercon-taminatedbyTCE.

Degradation of the Chlorinated Alkanes

ThedegradationofchlorinatedVOCsisfundamentallydifferentfromthatofBTEXcompounds(Wiedemeierandothers,1995).Thechlorinatedalkanescanbedegradedbyabioticprocessesthroughhydrolysisordehydrohalogenationorbybioticprocessesthroughreductivedechlorinationordichloroelimination.Thesedegradationprocessescanproceedundereitheraerobicoranaerobicconditions(figs.3–6;VogelandMcCarty,1987a;Vogel,1994).AccordingtoMcCarty(1997),111-TCAistheonlychlorinatedcompoundthatcanbedegradedingroundwaterwithin20yearsunderalllikelyground-wateroraquiferconditions.

Abiotic TransformationHydrolysisanddehydrohalogenationaretwoabiotic

processesthatmaydegradechlorinatedethanesundereitheraerobicoranaerobicconditions.Thetendencyforachlori-natedethanetodegradebyhydrolysisdependsontheratioofchlorinetocarbonatoms(fig.2)orthelocationofchlorineatomsonthenumber2carboninthecompound.Chlorinatedalkanesaremoreeasilyhydrolyzedwhenthechlorine-carbonratioislessthantwoorwhenchlorineatomsareonlylocatedonthenumber1carbonatom(VogelandMcCarty,1987b;Vogel,1994).Forexample,chloroethaneand111-TCAhavehalf-lifesthataremeasuredindaysormonths(Vogelandoth-ers,1987;Vogel,1994;table14).Conversely,themorechlo-rinatedethanessuchas1,1,1,2-tetrachloroethane(PCA)andthosewithchlorineatomsonthenumber2carbontendtohavehalf-lifesmeasuredindecadesorcenturies(table14).Dehy-drohalogenationistheremovalofoneortwohalogenatomsfromanalkane(VogelandMcCarty,1987a).Thedehydroha-logenationoftwochlorineatomsiscalleddichloroelimination.

Chenandothers(1996)showthatPCAcanbeabioticallytransformedtoTCEundermethanogenicconditions(fig.3).Inaddition,theabioticdegradationof111-TCAhasbeenwellstudiedinthescientificliterature(fig.4;Jeffersandothers,1989;McCartyandReinhard,1993;Chenandothers,1996;McCarty,1997).McCartyandReinhard(1993)indicatethatthetransformationof111-TCAbyhydrolysisisaboutfourtimesfasterthanbydehydrochlorination.Duringabioticdeg-radation,about80percentof111-TCAistransformedtoaceticacidbyhydrolysis(McCarty,1997),andtheremaining20per-centistransformedto11-DCEbydehydrochlorination(table7;VogelandMcCarty,1987b;McCarty,1997).Thepresenceof11-DCEincontaminatedgroundwaterisprobablytheresultofthedehydrochlorinationof111-TCA(McCarty,1997).


1,1,2-trichloroethene (TCE)

1,2-dichloro-ethene, cis

1,2-dichloro-ethene, trans

chloroethene(vinyl chloride)

ethene

ethane

chloroethane

1,1,2,2-tetrachloroethane (PCA) 1,1,2-trichloroethane (112-TCA)

1,2-dichloroethane (12-DCA)

(1) Chen and others, 1996(2) McCarty, 1997(3) Grostern and Edwards, 2006

(1)

(1)

(1)

(1)(1)

(1)(1)

(1)

(1, 3) (1)

(1, 3)

(1)

(1)

(2)

Microorganism catalyzing reactionDehalobactor sp.

Dehalococcoides sp.Dehalococcoides sp.

Dehalobactor sp.

Dehalobactor sp.

EXPLANATION

Literature reference describing reaction pathway

Proven microbe-catalyzed anaerobic dechlorination

Proven methanogenic dechlorination

Proven biotic dichloroelimination

Proven abiotic dehydrochlorination

Figure �. Laboratory-derived pathway for the abiotic degradation, anaerobic, and methanogenic biodegradation of 1,1,2,2-tetrachloroethane; 1,1,2-trichloroethene; and 1,1,2-trichloroethane (modified from Chen and others, 1996).

Table 1�. Laboratory half-lifes and by-products of the abiotic degradation (hydrolysis or dehydrohalogenation) of chlorinated alkane compounds detected in ground water.

[IUPAC,InternationalUnionofPureandAppliedChemistry;—,notapplicable]

Compound (IUPAC name)1 Degradation by-products Half-life Literature reference

chloroethane ethanol 44days Vogelandothers,1987

1,1-dichloroethane — 61years Jeffersandothers,1989

1,2-dichloroethane — 72years Jeffersandothers,1989

1,1,1-trichloroethane aceticacid;1,1-dichloroethane 1.1–2.5years MabeyandMill,1978;Jeffersandothers,1989;VogelandMcCarty,1987a,b

1,1,2-trichloroethane 1,1-dichloroethane 140years Jeffersandothers,1989

1,1,1,2-tetrachloroethane trichloroethene 47–380years MabeyandMill,1978;Jeffersandothers,1989

1,1,2,2-tetrachloroethane 1,1,2-trichloroethane;trichloroethene 146–292days MabeyandMill,1978;Jeffersandothers,1989



Figure �. Laboratory-derived pathway for the abiotic, aerobic, and anaerobic biodegradation of 1,1,1-trichloroethane (modified from Sands and others, 2005; Whittaker and others, 2005).

(1) Egli and others, 1987(2) Gälli and McCarty, 1989(3) De Best and others, 1999(4) Yagi and others, 1999(5) Oldenhuis and others, 1989(6) Newman and Wackett, 1997(7) Motosugi and others, 1982(8) Jun Oh, 2005(9) De Wever and others, 2000(10) McCarty, 1997

1,1,1-trichloroethane(Clostridium spp.,

Dehalobacter strain TCA1)

1,1-dichloroethane

chloroethane

acetaldehyde

(1, 2)

(2)

(3)

Anaerobic pathway

1,1,1-trichloroethane(Methylosinus trichosporium OB3b,

Mycobacterium spp. TA5, 27)

2,2,2-trichloroethanol

(4, 5)

2,2,2-trichloroacetate(Pseudomonas sp.)

oxalatecarbondioxide

(6)

(7)(8)

Aerobic pathway

2,2-dichloroacetate

(9)

1,1,1-trichloroethane

1,1-dichloroethene acetic acid

(10)

Abiotic pathway

EXPLANATION


Proven dehydrochlorination

Hydrolosis

Co-metabolism

Proven microbe-catalyzed

Proven abiotic hydrolysis

Methylosinustrichosporium Microorganism catalyzing reaction

Degradation of Selected Volatile Organic Compounds in Ground Water 19

Aerobic BiodegradationAccordingtothedegradationpathwayconstructedby

Sandsandothers(2005)andWhittakerandothers(2005),thedichloroethanesarenotaby-productof111-TCAor112-TCAbiodegradationunderaerobicconditions(fig.4).Apparently,theonlysourceof11-DCAand12-DCAviaadegradationpathwayisthereductivedechlorinationof111-TCAand112-TCA,respectively,underanaerobicconditions(figs.3and4).Underaerobicconditions,however,12-DCAcanbedegradedwhenusedasacarbonsourcebymicroorganisms.Theintermediateby-productofthisdegradationischloroetha-nol,whichisthenmineralizedtocarbondioxideandwater(fig.5;Stuckiandothers,1983;Janssenandothers,1985;Kimandothers,2000;Hageandothers,2001).

Anaerobic Biodegradation Whileresearchingthescientificliteraturefortheirreport,

Wiedemeierandothers(1998)didnotfindpublishedstudiesdescribinganaerobicbiodegradationofchlorinatedethanesingroundwater.SincethepublicationofWiedemeierandothers(1998),however,numerouspublishedstudiesdescribetheanaerobicbiodegradationofchlorinatedethanes.McCarty(1997)indicatesthatcarbontetrachloridewastransformedtochloroformunderdenitrifyingconditionsandmineralizedtocarbondioxideandwaterundersulfate-reducingcondi-tions(fig.6).AdamsonandParkin(1999)showthatunderanaerobicconditions,carbontetrachlorideand111-TCAtendtoinhibitthedegradationofeachother.AdamsonandParkin(1999)alsoshowthatcarbontetrachloridewasrapidlydegradedbyco-metabolismwhenacetatewasthecarbonsource.

Chenandothers(1996)describehowmethanogenicconditionsinamunicipalsludgedigesterallowed the degra-dationof PCAto112-TCA,and112-TCAto12-DCAthrough dehydrohalogenation(fig.3).DeBestandothers(1999)reportthatco-metabolictransformationsof112-TCAwill-occurundermethanogenicconditions.Inthisstudy,112-TCAwasdegradedtochloroethanewhensufficientamountsofthecarbonsourcewerepresent(fig.3).Thistransformationwasinhibitedbythepresenceofnitrate,butnotnitrite.

Dolfing(2000)discussesthethermodynamicsofreduc-tivedechlorinationduringthedegradationofchlorinatedhydrocarbonsandsuggeststhatfermentationofchloroethanestoethaneoracetatemaybeenergeticallymorefavorablethan“classic”dechlorinationreactions.Moreover,polychlorinatedethanesmaydegradepreferentiallybyreductivedechlorina-tionunderstronglyreducingconditions.Dichloroelimination,however,mayactuallybethedominantdegradationreactionforpolychlorinatedethanesbecausemoreenergyisavailabletomicroorganismsthanisavailableduringreductivedechlo-rination(Dolfing,2000).During anaerobicbiodegradation,themeanhalf-lifesofthechloroethanecompoundscanbeasshortasthreedays,inthecaseof111-TCA,oraslongas165days,inthecaseof12-DCA(table15).

Table 1�. Mean half-life in days for the anaerobic biodegradation of selected chlorinated alkane and alkene compounds.1

[(27),numberofsamplesusedtoderivethemeanvalue;—,notavailable)

Compound All studies Field/in situ studies

chloroethene 0.018(27) 0.0073(19)

1,2-dichloroethane 63-165(2) 63–165(2)

tetrachloroethene(PCE) 239–3,246(36) 239(16)

tetrachloromethane 47(19) 40(15)

1,1,1-trichloroethane 2.3–2.9(28) —

1,1,2-trichloroethane 47–139(1) —

trichloroethene(TCE) 1,210(78) 277(30)1AronsonandHoward,1997,p.111

1,2-dichloroethane, aerobic(xanthobacter autotrophicus GJ10,

Ancylobacter aquaticus Ad20, AD25, AD27)

2-chloroethanol

choroacetaldehyde

chloroacetic acid

glycolate

intermediarymetabolism

(1)

(2)

(3)

(4, 5)

(1) Verschueren and others, 1993(2) Xia and others, 1996(3) Liu and others, 1997(4) Janssen and others, 1985(5) Hisano and others, 1996

EXPLANATION


Proven microbe-catalyzed oxidation reaction

Microorganism catalyzing reactionxanthobacterautotrophicus

Figure �. Laboratory-derived pathway for the aerobic biodegradation of 1,2-dichloroethane (modified from Renhao, 2005).

�0 Description, Properties, and Degradation of Selected VOCs Detected in Ground Water

carbon tetrachloride

Pathway 1 Pathway 2 Pathway 3

(Pseudomonas stutzeri KC, Methanosarcina barkeri,Desulfobacterium autotrophicum, Moorella thermoacetica,

Methanoabacterium thermoautotrophicum)

chloroform

dichloromethane

methyl chloride

methane

C1 metabolic cycle

carbonmonoxide

(Methylococcuscapsulatus;

Pseudomonasaeruginosa)

carbondioxide

formate

phosgenethiophosgene

carbondioxide

(1)

(1)

(1)

(1)

(1)

(1)

(2)

(2) (1)

(3) (4)

(1)

(1) Lee and others, 1999(2) Krone and others, 1991(3) Eggen and others, 1991(4) Lamzin and others, 1992

thiophosgene

Pseudomonasstutzeri

EXPLANATION

Proven microbe-catalyzed dechlorination

Postulated intermediate compound

Postulated microbe-catalyzed reductive pathway


Proven microbe-catalyzed reductive pathway

Microorganism catalyzing reaction

Figure �. Laboratory-derived pathways for the anaerobic biodegradation of tetrachloromethane (carbon tetrachloride; modified from Ma and others, 2005; Sands and others, 2005).

Degradation of the Chlorinated Alkenes

Theprimarydegradationofthemostcommonchlorinatedalkenesismicrobialreductivedechlorinationunderanaerobicconditions.However,biodegradationofcertainchlorinatedcompounds—suchastrichloroethene,thedichloroethenes,vinylchloride,orchloroethane—canalsoproceedviaoxida-tivepathwaysunderaerobicconditions.Twoforms(isomers)ofdichloroetheneoccuringroundwateraschemicalby-prod-uctsofPCEandTCEbiodegradation(Wiedemeierandothers,1998;Olaniranandothers,2004).AbioticdegradationofPCAtoTCEcanoccurinPCA-contaminatedgroundwater(fig.3;Chenandothers,1996).

Aerobic Biodegradation

Severalstudieshaveshownthatchlorinatedethenes,withtheexceptionofPCE,candegradeunderaerobicconditionsbyoxidation(HartmansandDeBont,1992;Klierandothers,1999;HopkinsandMcCarty,1995;Colemanandothers,2002)andbyco-metabolicprocesses(MurrayandRichardson,1993;Vogel,1994;McCartyandSemprini,1994).Studiesdescrib-ingthedegradationofPCEunderaerobicconditionswerenotfoundinthepeer-reviewedliterature.Inonestudy,aerobicbiodegradationofPCEwasnotmeasurablebeyondanalyti-calprecisionafter700daysofincubation(Robertsandothers,1986).Furthermore,Aronsonandothers(1999)indicatethat

Degradation of Selected Volatile Organic Compounds in Ground Water �1

PCEisnotdegradedwhendissolvedoxygen(DO)isgreaterthan1.5mg/L,theapproximateboundarybetweenaerobicandanaerobicconditions(StummandMorgan,1996).Chenandothers(1996)suggestthestructureandoxidativestateofPCEpreventsitsaerobicdegradationinwater.

Accordingtotheaerobicbiodegradationpathwaycon-structedbyWhittakerandothers(2005),thedichloroethenesarenotaby-productofTCEdegradationunderaerobiccondi-tions(fig.4).Rather,TCEisdegradedalongthreedifferentpathwaysbydifferentmicroorganisms(fig.7).Thesepathwaysdonotformanyofthedichlorothenecompoundsandtheonlyapparentsourceof12-DCEisbythereductivedechlorinationofTCEunderanaerobicconditions(figs.3and8).Thecom-pounds12-DCEandVC,however,canbedegradedunderaero-bicconditionsbymicroorganismsutilizingthecompoundsasaprimarycarbonsource(fig.5;BradleyandChapelle,1998).

AlthoughPCEisnotknowntodegradethroughco-metabolismunderaerobicconditions,co-metabolismisknowntodegradeTCE,thedichloroethenes,andVC.Therateofco-metabolismincreasesasthedegreeofchlorinationdecreasesontheethenemolecule(Vogel,1994).Duringaerobicco-metabolism,thechlorinatedalkeneisindirectlydechlorinatedbyoxygenaseenzymesproducedwhenmicroorganismsuseothercompounds,suchasBTEXcompounds,asacarbonsource(Wiedemeierandothers,1998).Theco-metabolicdeg-radationofTCE,however,tendstobelimitedtolowconcen-trationsofTCEbecausehighconcentrationsinthemilligramperliterrangearetoxictomicrobescatalyzingthisreaction(Wiedemeierandothers,1998).InfieldstudiesbyHopkinsandMcCarty(1995),VCisshowntodegradebyco-metabo-lismunderaerobicconditionswhenphenolandtoluenewereusedasacarbonsource.

trichloroethene (TCE)(Methylosinus trichosporium OB3b)

trichloroethene (TCE)(Burkholderia cepacia G4)

trichloroethene (TCE)(Pseudomonas putida F1)

chloral hydrate

trichloroacetate(Pseudomonas sp.)

trichloroethene epoxide glyoxylate

formatetrichloroethanol

dichloroacetate(Pseudomonas sp.)

glyoxylate

carbon monoxide(Methylococcus capsulatus,Pseudomonas aeruginosa)

carbon dioxide

oxalatecarbondioxide

(1)

(2)

(2)

(2)

(3)(4)

(1) (8)(8)(5)

(1) (6)

(6)

(6)

(7)

(1) Rosenzweig and others, 1993(2) Newman and Wackett, 1991(3) Motosugi and others, 1982(4) Jun Oh, 2005(5) Newman and Wackett, 1995(6) Newman and Wackett, 1997(7) Weightman and others, 1985(8) Li and Wackett, 1992(9) Eggen and others, 1991

(9)

C1 metabolic cycle

EXPLANATION


Dechlorination

Hydrolysis


Methylosinustrichosporium Microorganism catalyzing reaction

Figure �. Laboratory-derived pathways for the aerobic biodegradation of trichloroethene (modified from Whittaker and others, 2005).

�� Description, Properties, and Degradation of Selected VOCs Detected in Ground Water

Anaerobic BiodegradationManylaboratoryandfieldstudieshaveshownthat

microorganismsdegradechlorinatedethenesunderanaerobicconditions(Bouwerandothers,1981;Bouwer,1994,Dolfing,2000).Groundwaterisconsideredanoxicwhenthedissolvedoxygenconcentrationfallsbelow1.0–1.5mg/L(StummandMorgan,1996;Christensenandothers,2000).Underanoxicconditions,anaerobicorfacultativemicrobeswillusenitrateasanelectronacceptor,followedbyiron(III),thensulfate,andfinallycarbondioxide(methanogenesis;Chapelleandoth-ers,1995;Wiedemeierandothers(1998).Astheconcentrationofeachelectronacceptorsequentiallydecreases,theredoxpotentialofthegroundwaterbecomesgreater(morenegative)andbiodegradationbyreductivedechlorinationisfavored.

Anaerobicconditionsingroundwatercanbedeterminedbymeasuringtheverticalandspatialconcentrationsofoxy-gen,iron(II),manganese(II),hydrogensulfide,ormethaneingroundwaterandusingthatdataasaqualitativeguidetotheredoxstatus(StummandMorgan,1996;Christensenandothers,2000).Othermeasurementsofanaerobicconditionsinvolvingmicroorganismbiomarkersincludevolatilefattyacids,ester-linkedphospholipidfattyacid(PLFA),deoxyribo-nucleicacid(DNA),andribonucleicacid(RNA)probes,andTEAPbioassay(Christensenandothers,2000).Thereductionofiron(III)toiron(II),manganese(IV)tomanganese(II),sulfatetohydrogensulfide,andcarbondioxidetomethaneduringthemicrobialreductionofchlorinatedVOCscanhaveamajorinfluenceonthedistributionofiron(II),manganese(II),hydrogensulfide,andmethaneconcentrationsingroundwater(StummandMorgan,1996;Lovley,1991;Higgoandothers,1996;Braun,2004).

Thehighlychlorinatedalkenesarecommonlyusedaselectronacceptorsduringanaerobicbiodegradationandarereducedintheprocess(Vogelandothers,1987).TheprimaryanaerobicprocessdrivingdegradationofCVOCs,exceptVC,isreductivedechlorination(figs.3and8;Bouwerandothers,1981;Bouwer,1994).TetrachloroetheneandTCEarethemostsusceptibletoreductivedechlorinationbecausetheyarethemostoxidizedofthechlorinatedethenes;however,themorereduced(leastoxidized)degradationby-productssuchasthedichloroethenesandvinylchloridearelesspronetoreductivedechlorination.Themainby-productofanaerobicbiodegra-dationofthepolychlorinatedethenesisVC(fig.8),whichismoretoxicthananyoftheparentcompounds(AgencyforToxicSubstancesandDiseaseRegistry,2004).Therateofreductivedechlorinationtendstodecreaseasthereduc-tivedechlorinationofdaughterproductsproceeds(VogelandMcCarty,1985;Bouwer,1994).MurrayandRichardson(1993)suggestthattheinverserelationbetweenthedegreeofchlorinationandtherateofreductivedechlorinationmayexplaintheaccumulationof12-DCEandVCinanoxicgroundwatercontaminatedwithPCEandTCE.Inaddition,theanaerobicreductionofVCtoetheneisslowandinefficientunderweakreducingconditions,whichfavorsthepersistenceofVCinanoxicgroundwater(FreedmanandGossett,1989).

Reductivedechlorinationhasbeendemonstratedundernitrate-andiron-reducingconditions(Wiedemeierandothers,1998).ReductivedechlorinationoftheCVOCs,however,maybemorerapidandmoreefficientwhenoxidation-reduction(redox)conditionsarebelownitrate-reducinglevels(Azad-pour-Keeleyandothers,1999).Sulfate-reducingandmetha-nogenicground-waterconditionscreateanenvironmentthatfacilitatesnotonlybiodegradationforthegreatestnumberofCVOCs,butalsomorerapidbiodegradationrates(Bouwer,1994).ReductivedechlorinationofDCEandVCismostapparentundersulfatereducingandmethanogeniccondi-tions(Wiedemeierandothers,1998).Anaerobicbiodegradationratesforthechlorinatedalkenescanbeasshortas45minutes,inthecaseofVC,toaslongas9yearsforPCE(table15).

(1) Neumann and others, 1996(2) Maymo-Gatell and others, 1997(3) Magnuson and others, 1998

tetrachloroethene (PCE)(Dehalococcoides ethenogenes 195,

Dehalospirillum multivorans, Sporomusa ovata,Dehalobacter restrictus TEA, Desulfitobacterium sp. PCE-S)

trichloroethene (TCE)(Dehalococcoides ethenogenes 195)

cis-1,2-dichloroethene trans-1,2-dichloroethene

chloroethene(vinyl chloride)

ethene

(1)

(2) (2)

(2) (2)

(2)

(3)

EXPLANATION


Proven microbe-catalyzed dechlorination

Microorganism catalyzing reactionDehalococcoidesethenogenes

Figure 8. Laboratory-derived pathway for the anaerobic biodegradation of tetrachloroethene (modified from Ellis and Anderson, 2005).

Degradation of Selected Volatile Organic Compounds in Ground Water ��

Degradation of the Chlorinated BenzenesSeveralstudieshaveshownthatchlorinatedbenzenecom-

poundscontaininguptofourchlorineatomscanbedegradedbymicroorganismsunderaerobicconditions(ReinekeandKnackmuss,1984;SpainandNishino,1987;Sanderandoth-ers,1991).Underaerobicconditions,1,2,4-trichlorobenzene(124-TCB;Haiglerandothers,1988)andchlorobenzene(CB;Sanderandothers,1991)areusedasaprimarycarbonsourceduringbiodegradationbymicroorganismssuchasBurkholdiaandRhodococcusspecies(RappandGabriel-Jürgens,2003).Duringbiodegradation,thesecompoundsarecompletelymineralizedtocarbondioxide(CO

2)(vandeMeerandothers,

1991).RappandGabriel-Jürgens(2003)alsoindicatethatallofthedichlorobenzeneisomerswerebiodegradedbytheRhodococcusbacterium.Thebiodegradationpathwaysfor124-TCB,14-DCB,12-DCB,andCB,underaerobiccondi-tionsareshowninfigures9to11,respectively.Thesepathwaysaresimilartothatofbenzene,exceptthatonechlorineatomiseventuallyeliminatedthroughhydroxylationofthechlorinatedbenzenetoformachlorocatechol,thenorthocleavageofthebenzenering(vanderMeerandothers,1998).

Calculatedandpublisheddegradationhalf-lifesforthechlorobenzenesunderaerobicconditionsareshownintable16.Thecompounds124-TCB,12-DCB,andCBlose50percentoftheirinitialmasswithin180days(table16).Conversely,DermietzelandVieth(2002)showthatchlorobenzenewasrapidlymineralizedtoCO

2inlaboratoryandinsitumicrocosm

studies,withcompletemineralizationrangingfrom8hourstoabout17days.Inaddition,thecompound14-DCBwascompletelymineralizedwithin25days.Nevertheless,undertheaerobicconditionsofDermietzelandVieth(2002)study,124-TCB,12-DCB,and13-DCBwereonlypartiallydegradedafter25days.Inanotherlaboratory-microcosmstudybyMon-ferranandothers(2005),allisomersofDCBweremineralizedtoCO

2within2daysbytheaerobeAcidovorax avenae.

AlthoughWiedemeirandothers(1998)indicatethatfewstudiesexistedthatdescribedtheanaerobicdegradationofthechlorobenzenecompounds,astudybyRamanandandothers(1993)didsuggestthat124-TCBcouldbebiodegradedtochlo-robenzenewith14-DCBasanintermediatecompoundunderanaerobicconditions.Moreover,Middeldorpandothers(1997)showthat124-TCBwasreductivelydechlorinatedto14-DCB,thentochlorobenzeneinamethanogeniclaboratorymicrocosminwhichchlorobenzene-contaminatedsedimentwasenrichedwithlactate,glucose,andethanol.Thesecompoundsservedascarbonsources.Furthermore,themicrobialconsortiafacilitat-ingthedechlorinationof124-TCBalsowasabletodegradeisomersoftetrachlorobenzenetootherisomersofTCBand12-DCB.Morerecentstudies showthatastrainofthebacterium,Dehalococcoides,canreductivelydechlorinate124-TCBunderanaerobicconditions(Holscherandothers,2003;Grieblerandothers,2004a).Inaddition,Adrianandothers(1998)suggestthatfermentationistheprimarydegradationprocessforthechlorobenzenesunderanaerobicconditions.Thisstudyalsoshowedthattheco-metabolismof124-TCBwasinhibitedbythepresenceofsulfate,sulfite,andmolybdate.

1,2,4-trichlorobenzene(Burkholderia sp. P51, PS12)

3,4,6-trichlorocatechol

2,5-dichloro-carboxymethylenebut-2-en-4-olide

2-chloro-3-oxoadipate

(1)

(1)

(3)

(1)

(2)

(1) van der Meer and others, 1991(2) Kasberg and others, 1995(3) Reineke and Knackmuss, 1988(4) Middeldorp and others, 1997(5) National Institute for Resources and Environment, 2001

3,4,6-trichloro-cis-1,2-dihydroxy-cyclohexa-3,5-diene

2,3,5-trichloro-cis,cis-muconate

(1)

2,5-dichloro-4-oxohex-2-enedioate

chloroacetatesuccinate

1,2-dichloroethanepathway (fig. 5)

1,2,4-trichlorobenzenemethanogenic microbial consortium

1,4-dichloro-benzene

1,2-dichloro-benzene

(4) (5)

(5)

60 percent 10 percent

chlorobenzene

(4)

chlorobenzenepathway(fig. 11)

EXPLANATION



Burkholderia sp. Microorganism catalyzing reaction

Anaerobic pathwayAerobic pathway

Oxidation reaction

Dechlorination

Hydrolysis

Figure 9. Laboratory-derived pathways for the aerobic and anaerobic biodegradation of 1,2,4-trichlorobenzene (modified from Yao, 2006).


1,4-dichlorobenzene(Xanthobacter flavus 14pl ;Alcaligenes sp. strain A175;

Pseudomonas sp.)

3,6-dichloro-cis- 1,2-dihydroxycyclohexa-3,5-diene

3,6-dichlorocatechol

2,5-dichloro-cis,cis-muconate

trans-2-chlorodienelactone

(1)

(1)

(1)

(2)

(1) Spiess and others, 1995(2) Sommer and Görisch, 1997

EXPLANATION


Proven microbe-catalyzed oxidation reaction

Alcaligenes sp. Microorganism catalyzing reaction

Figure 10. Laboratory-derived pathway for the aerobic biodegradation of 1,4-dichlorobenzene (modified from Liu, 2006).

Furthermore,Ramanandandothers(1993)showthat124-TCBhaddeclinedby63percentwithin30daysunderanaerobicconditions.DermietzelandVieth(2002)showthattheanaerobicbiodegradationof14-DCBwasmarkedlyslowerunderiron-reducingconditionsthanunderaerobicconditions.Ingeneral,itappearsthatthebiodegradationofthechlorinatedbenzenesisslowerunderanaerobicthanunderaerobicconditions.

Degradation of the Gasoline Compounds

Laboratoryandfieldstudieshaveshownthatmicroorgan-ismsmediatethedegradation(biodegradation)ofthecommongasolinecompounds(MTBEandBTEX)underbothaerobicandanaerobicconditions.Aerobicmicroorganismsread-ilyoxidizeBTEXcompoundswhileusingthemasprimarysubstrates.ThebiodegradationofBTEXcompoundsundervariousredoxconditionsiswelldocumentedinthescientificliterature(VogelandGrbić-Galić,1986;Kuhnandothers,1988;LovleyandLonergan,1990;Evansandothers,1991a,b;Hutchinsandothers,1991;Rabusandothers,1993;EdwardsandGrbić-Galić,1994;Friesandothers,1994;RabusandWiddel,1995;Andersonandothers,1998).

AlthoughearlystudiesconcludedthatMTBEwasrecalcitranttoaerobicbiodegradation(Squillaceandothers,1997),morerecentstudiesshowthat,onceinitiated,theaerobicbiodegradationofMTBEisrelativelyrapid(Deebandothers,2000),butmarkedlyslowerthanBTEXdegradation.Inaddition,theanaerobicbiodegradationofMTBEisknowntoproceed,althoughslowly,underavarietyofredoxcondi-tions.Untilrecently,however,littlewasknownaboutspecificpathwaysinvolvedintheanaerobicdegradationofMTBE.

Aerobic Biodegradation of BTEX Compounds Laboratoryandfieldstudiesshowthatmicroorgan-

ismsmediatethebiodegradationofBTEXcompoundsunderaerobicconditions(table17;Aronsonandothers,1999).ThemicrobiallycatalyzedoxidationreactionbetweendissolvedoxygenandBTEXisthermodynamicallyfavoredbecauseBTEXcompoundsareinahighlyreducedstateandthepre-ferredterminalelectronacceptor(TEA)isoxygen(Brownandothers,1996).ThemicrobiallycatalyzedoxidationofBTEXcompoundsrequires3.1milligramsperliter(mg/L)ofdis-solvedoxygen(DO)to1mg/LofaBTEXcompound(Aron-sonandHoward,1997).SomestudiesshowthattherateofbiodegradationtendstoslowwhenDOconcentrationsarelessthanabout1–2partspermillion(ppm;equaltomilligramsperliter,mg/L;Chiangandothers,1989;Salanitro,1993).DuringlaboratorystudiesinwhichtheinitialDOconcentra-tionwasatleast8mg/L,individualBTEXcompoundsoraBTEXmixturebiodegradedrapidlytolowconcentrationsuntiltheDOconcentrationwaslessthan2mg/L;Atthisthreshold,biodegradationwasratelimited,ratherthansubstratelimited,becauseofthelowDOconcentration(Salanitro,1993).


Chlorobenzene(Pseudomonas sp.

P51, JS150, RHO1, JS6)

1,2-dichlorobenzene(Pseudomonas sp.)

3-chloro-cis-1,2-dihydroxy-cyclohexa-3,5-diene

3-chlorocatechol

2-chloro-cis,cis-muconate 3-chloro-2-hydroxymuconic

semialdehyde

trans-4-carboxymethylene-but-2-en-4-olide

maleylacetate

3-oxoadipate

(1)o-dichlorobenzine dihydrodiol

(8)

3,4-dichlorocatechol

(8)

2,3-dichloro-cis-cis-muconic acid

(8)

5-chloro-4-carboxy-methylene-but-2-en-4-olide

(8)

5-chloromaleylacetic acid

(8)

trans-2-chlorodienelactone2-chloromaleylacetate

(8)

(8)(9)

(2)

(3) (4)

(5)

(6)

(7)

(1) Werlen and others, 1996(2) Mason and Geary, 1990(3) Broderick and O’Halloran, 1991(4) Riegert and others, 1998(5) Hammer and others, 1993(6) Pathak and Ollis, 1990(7) Kaschabek and Reineke, 1995(8) Haigler and others, 1988(9) Vollmer and others, 1993

EXPLANATION




Pseudomonas sp.

Oxidation reaction

Dechlorination

Hydrolysis

Hydration

Figure 11. Laboratory-derived pathway for the aerobic biodegradation of chlorobenzene and 1,2-dichlorobenzene (modified from McLeish, 2005).

AerobicmicroorganismsreadilyoxidizeBTEXcom-poundswhileusingthemasprimarysubstrates.Theoxida-tionofBTEXcompoundscanproceedviaseveralpathways(figs.12–16).Inonelaboratorycolumnstudy,methanolwasaddedtoaBTEXmixturetoidentifypossibleco-metabolicpathways.ThemethanolwasnotusedastheprimarysubstrateandappearedtodepressthebiodegradationofBTEXcom-pounds(Hubbardandothers,1994).ThisstudyshowedthatBTEXdegradationwasnotaresultofco-metabolism.Ben-zenehasbeenshowntodegradecompletelytocarbondioxide(mineralization;Gibsonandothers,1968; GibsonandSubra-manian,1984;EdwardsandGrbić-Galić,1992).Morerecentlaboratoryexperimentsshowthatcatecholisanintermediatecompoundinthebenzenepathway(fig.12).Fiveseparate

degradationpathwayshavebeenidentifiedfortolueneunderaerobicconditions(fig.13).Oneofthesepathwayssharesacommonintermediatecompound(3-methylcatechol)withthedegradationofo-andm-xylene(figs.12and13).Thepathwayforp-xylenefollowsasimilarpattern,butdiffersintheinter-mediatecompoundsformed.Thisdifferenceiscausedbythepositionofthemethylgrouponthebenzeneringofp-xylene(fig.14).Theaerobicbiodegradationofethylbenzenefollowsthreepathwaysdependingonthemicroorganismusingethyl-benzeneasitscarbonsource(fig.15).Basedonlaboratoryandfieldmicrocosmstudies,biodegradationofBTEXunderaero-bicconditionsismorerapidingasoline-contaminatedaquifersedimentsthaninuncontaminatedaquifersediments(Aronsonandothers,1999;table17).


AnationwidesurveyofVOCsingroundwatershowedthattoluene,representingBTEXcompounds,wasdetectedmorefrequentlyinoxicratherthaninanoxicgroundwater(SquillaceandMoran,2006).Inotherstudies,thelossofBTEXcompoundsalongground-waterflowpathswasinverselyrelatedtodissolved-oxygenconcentration,indicat-ingthatmicrobialactivity(respiration)wasrelatedtoBTEXdegradation(Donaldsonandothers,1990;HuesemannandTruex,1996).

Moraschandothers(2001,2002),usingstableisotopefractionationdata,concludedthatquantifyingaerobicmicro-bialdegradationofBTEXinoxicenvironmentsmaynotbepossible.Moreover,laboratorystudieshaveshownthatethylbenzenecaninhibitthemicrobialdegradationofbenzene,toluene,andthexylenesanddoessountilalloftheethylben-zeneisdegraded(DeebandAlvarez-Cohen,2000).

Anaerobic Biodegradation of BTEX Compounds

Duringanaerobicbiodegradation,BTEXcompoundsareusedmetabolicallyaselectrondonors(carbonsource,primarysubstrate)byselectmicrobialpopulationstoproducetheenergyforcellgrowth(AronsonandHoward,1997).BTEXdegradationcanbelimitedbytheavailabilityofterminalelectronacceptorssuchasnitrate,sulfate,carbondioxide,oriron(III)intheaquifer(Lovleyandothers,1989;Lovleyandothers,1995).Theseelectronacceptors,however,commonlyexistingroundwateratsufficientlevelsforthesereactionstoproceed(LovleyandLonergan,1990;Kuhnandothers,1988).Figure16showstheanaerobicbiodegradationpathwaysforBTEXcompounds.

Anaerobicbiodegradationofbenzeneappearstobemoreaquiferspecificthanthatfortheothermonoaromatichydro-carbons.Currentdataindicatesthatbiodegradationmaynotoccuratallsites(AronsonandHoward,1997).Someofthesestudiesshowthatbenzeneresistsanaerobicmetabolisminthefield(Reinhardandothers,1984;Barbaroandothers,1992)andinlaboratoryenrichmentsestablishedwithsewagesludge,aquifersediments,andcontaminatedsoils(Krumholzandoth-ers,1996;Barbaroandothers,1992).

Conversely,severalground-waterstudieshaveshownthatBTEXdegradationratesdeclineinasequencefrommildlyreducingconditions(nitratereductionzone,Hutchinsandoth-ers,1991)tostronglyreducingconditions(methanogenesis)inshallowaquifers(Kazumiandothers,1997;Luandothers,1999;RoychoudhuryandMerrett,2005).Furthermore,otherstudiesshowthatwhenconditionsarefavorable,benzene(andotherBTEXcompounds)canbeoxidizedtocarbondioxideunderhighlyreducingconditions.Forexample,benzenewasrapidlymineralizedundersulfate-reducingconditionsinmarineandfreshwatersediments,andinaquifersediments(LovleyandLonergan,1990;EdwardsandGrbić-Galić,1992;Lovleyandothers,1995;Phelpsandothers,1996;Coatesandothers,1996a,b;WeinerandLovley,1998).

TherateofanaerobicbiodegradationofBTEXcom-poundswasquickestundersulfate-reducingconditionsinlaboratoryandfield/insitustudies(Bellerandothers,1992a,b;table18).TheanaerobicbiodegradationofBTEXcompoundsingroundwaterwasconclusivelyshowninsitubyGrieblerandothers(2004b)usingcompound-specificisotopeanaly-sisandsignaturemetabolitesanalysis.Figure16showstheanaerobicpathwayforBTEXcompoundsdevelopedfromtheintermediatecompoundsidentifiedinground-watersamplesbyGrieblerandothers(2004b).

Table 1�. Laboratory or environmental half-lifes and by-products for the aerobic and anaerobic biodegradation of selected chlorinated benzene compounds detected in ground water.

[IUPAC,InternationalUnionofPureandAppliedChemistry;CO2,carbondioxide;DCB,dichlorobenzene]

Compound (IUPAC name)1

Degradation by-products

Half-life (days)

Literature reference

Aerobic conditions

chlorobenzene 3-chlorocatechol,CO2

69–150 Rathbun,1998;McLeish,2005

1,2-dichlorobenzene chlorobenzene 28–180 Rathbun,1998

1,4-dichlorobenzene chlorobenzene 28–180 Rathbun,1998

1,2,4-trichlorobenzene succinate,chloroacetate 28–180 Rathbun,1998;Renhao,2005;Yao,2006

Anaerobic conditions

chlorobenzene CO2

280–580 Rathbun,1998;Monferranandothers,2005

1,2-dichlorobenzene CO2

119–722 Rathbun,1998

1,4-dichlorobenzene chlorobenzene 112–722 Rathbun,1998;Yao,2006

1,2,4-trichlorobenzene 1,4-DCB,chlorobenzene 112–722 Rathbun,1998;Yao,20061InternationalUnionofPureandAppliedChemistry,2006


3-methylcatechol +carbon dioxide (CO2)

cis,cis-2-hydroxy-6-oxohept-2,4-dienoate

cis-2-hydroxypenta-2,4-dienoate 4-hydroxy-2-oxovalerate

acetaldehyde

o-xylene(Burkholderia cepacia MB2)

2-methylbenzyl alcohol

2-methylbenzaldehyde

o-methylbenzoate

1,2-dihydroxy-6-methycyclohexa-3,5-dienecarboxylate

m-xylene(Sphingomonas yanoikutae B1,

Pseudomonas putida mt-2)

3-methylbenzyl alcohol

3-methylbenzaldehyde

m-methylbenzoate

1,2-dihydroxy-3-methycyclohexa-3,5-dienecarboxylate

(7)

(7)

(7)

(8)

(8)

(9)

(6)(6)

(10)

(11)

(12)

(13)

(14)

(8) Higson and Focht, 1992(9) Kukor and Olsen, 1991(10) Suzuki and others, 1991(11) Katagiri and others, 1967(12) Chalmers and Fewson, 1989(13) Harayama and others, 1986(14) Neidle and others, 1992

EXPLANATION


benzene

cis-dihydrobenzenediol

catechol

2-hydroxy-cis,cis-muconate semialdehyde

(1)

(2)

formate

cis,cis-muconate

(3)(4)

(5)

(5)

pyruvate

(1) Zamanian and Mason, 1987(2) Mason and Geary, 1990(3) Ngai and others 1990(4) Cerdan and others 1994(5) Horn and others, 1991(6) Lau and others, 1994(7) Jorgensen and others, 1995

(6)(6)


Burkholderiacepacia


Oxidation reaction

Hydrolysis

Hydration

Reaction, unspecified

Figure 1�. Laboratory-derived pathways for the aerobic biodegradation of benzene, o-, and m-xylene (modified from Hyatt and Jun Oh, 2005; Jun Oh, 2005).


toluene(Pseudomonas mendocina)

toluene(Pseudomonas pickettii)

toluene(Pseudomonas putida)

benzyl alcohol 2-hydroxy-toluene toluene-cis-dihydrodiol3-hydroxy-toluene(m-cresol)

4-hydroxy-toluene

3-methylcatechol + carbon dioxide (CO2)

cis, cis-2-hydroxy-6-oxohept-2,4-dienoate

cis-2-hydroxypenta-2,4-dienoate

4-hydroxy-2-oxovalerate

acetaldehyde + pyruvate

benzaldehyde

benzoate

benzoatepathway

toluene(Pseudomonas mendocina)

4-hydroxy-benzaldehyde

4-hydroxy-benzoate

vanillin pathway

m-cresol pathway(fig. 17)

(1)

(2)

(3)

(4)

(4)

(5)

(5)

(6)

(7)

(8)

(8)

(9)

(10)

(11)

(12)

(13)

(1) Shaw and Harayama, 1992(2) Shaw and others, 1993(3) Inoue and others, 1995(4) Shields and others, 1995(5) Olsen and others, 1994(6) Kukor and Olsen, 1991(7) Menn and others, 1991

toluene

EXPLANATION

Literature reference describing reaction pathwayProven microbe-catalyzed

(8) Lau and others, 1994(9) Wackett and others, 1988(10) Simpson and others, 1987(11) Yen and others, 1991(12) McIntire and others, 1986(13) Bossert and others, 1989Microorganism catalyzing

reactionPseudomonas

mendocina

Oxidation reaction

Hydrolysis

Hydration

Figure 1�. Laboratory-derived pathways for the aerobic biodegradation of toluene (modified from Wackett and Zeng, 2004).


Aerobic Biodegradation of Methyl Tert-butyl Ether

Thecompoundmethyltert-butylethercontainsetherbondsandbranchedhydrocarbonskeletons(tert-butylbranch)thatarecommontocompoundsthatpersistintheenvironment(Smithandothers,2003;Alexander,1973).AlthoughearlystudiesconcludedthatMTBEwasrecalcitranttobiodegrada-tion(Squillaceandothers,1997),morerecentstudiesshowthat,onceinitiated,theaerobicbiodegradationofMTBEisrelativelyrapid(Deebandothers,2000),butmarkedlyslowerthanBTEXdegradation(table17).MTBEmayappeartopersistincontaminatedgroundwaterifground-waterstudiesareconcludedtooquickly,especiallyinareaswhereMTBEisanewcontaminant.ThistimelagbeforedegradationbeginsisthetimeittakesthemicroorganismsintheaquifertoadaptandbegintouseMTBEasacarbonsource(DrogosandDiaz,2000;Wilsonandothers,2000,2005).Wilsonandothers(2005),usingdatafromvariousstudies,showthatmicroorgan-ismscapableofdegradingMTBEtakefrom10to500timeslongertodoubletheirpopulationthandothosemicrobesthatdegradeBTEXcompounds.Therefore,thecapacityforthenaturalattenuationofMTBEdependsontheageofthecontaminationandthepresenceofmicroorganismscapableofassimilatingMTBE.

AlthoughsomestudiesconcludedthatMTBEdegradesslowlyinaerobicenvironments(Squillaceandothers,1997),othermorerecentstudiesshowthatMTBEiseasilydegradedundertheproperconditions.Forexample,inalaboratorystudyoflakeandstreambedsedimentscollectedfrom11 sitesacrosstheUnitedStates,MTBEcompletelydegradedwithin50days(Bradleyandothers,1999,2001).AstudybyLand-meyerandothers(2001)clearlyshowsthatMTBEwasrecalcitrantunderanaerobicconditionsduring2weeksofmonitoring,butrapidlydegradedwhenoxygenwasaddedtoasmall,discreteflowpathinshallowgroundwater.

AlthoughthreedifferentbacteriaareabletoaerobicallydegradeMTBEviatwodifferentpathways(fig.17;PedersenandEssenberg,2005),theexactmechanismsdrivingMTBEdegradationarenotwellknown.Tert-butylalcohol(TBA)isacommonlydetectedby-productofaerobicMTBEdegradation(Steffanandothers,1997).AerobicbiodegradationratesforMTBEaredifficulttofindintheliterature,butthosethatarepublishedindicatethattheratesaresubstantiallyslowerthanthoseforBTEXcompounds(table17).SomestudiesindicatethatthedegradationofMTBEmaybeinhibitedbythepres-enceofBTEXcompounds(DeebandAlvarez-Cohen,2000);however,othersindicatethatBTEXcompoundsdonotinhibitMTBEdegradation(Aronsonandothers,1999;DrogosandDiaz,2000;Kaneandothers,2001;Sedranandothers,2002).

p-xylene(Pseudomonas putida mt-2)

p-methylbenzyl alcohol

p-tolualdehyde

p-toluate

4-methycyclohexa-3, 5-diene-1, 2-cis-diol-1-carboxylic acid

4-methylcatechol

2-hydroxy-5-methyl-cis,cis-muconic semialdehyde

(1) Shaw and Harayama, 1992(2) Biegert and others, 1995(3) Shaw and Harayama, 1990(4) Walsh and others, 1983(5) Whited and others, 1986(6) Cerdan and others, 1994

3-methyl-cis, cis-hexadienedioate

4-methylmucono-lactone2-oxohex-trans-4-enoate

4-hydroxy-2-oxohexanoate

pyruvate

(1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(11)

propanol

(9)

EXPLANATION


(7) Diaz and Timmis, 1995(8) Harayama and others, 1989(9) Platt and others, 1995(10) Murakami and others, 1997(11) Helin and others, 1995

Pseudomonasputida Microorganism catalyzing reaction

Proven microbe-catalyzedOxidation reaction

Hydrolysis

Hydration


Figure 1�. Laboratory-derived pathways for the aerobic biodegradation of p-xylene (modified from Mili and Stephens, 2006).


ethylbenzene(Peudomonas putida 39/D,

Pseudomonas sp. NCIB 10643)

ethylbenzene(Pseudomonas sp. NCIB 9816-4)

cis-2,3-dihydroxy-2,3-dihydroethylbenzene

2,3-dihydroxyethylbenzene

2-hydroxy-6-oxoocta-2,4-dienoate

propanoate cis-2-hydroxypenta-2,4-dienoate


acetaldehyde pyruvate

styrene (S)-1-phenylethanol

acetophenone

2-hydroxyaceto-phenone

(1)

(1)

(2)

(2) (2)

(3)

(3)(3)

To thestyrenepathway(fig. 19)

(1) Gibson and others, 1973(2) Jindrova and others, 2002(3) Lau and others, 1994(4) Lee and Gibson, 1996

(4) (4)

(4)

(4)

EXPLANATION


Microorganism catalyzing reactionPeudomonas putida


Hydrolysis

Hydration

Figure 1�. Laboratory-derived pathway for the aerobic biodegradation of ethylbenzene (modified from McLeish, 2005).

Inasix-stateground-waterstudy,Kolhatkarandothers(2000)observedthedegradationofMTBEandTBAinanoxiczonesnear76gasstations.Usingdatafromfourofthosesites,degradationrateswerecalculatedforMTBEandTBA.Thesedegradationratesrangedfrom0.0011to0.0271day–1forMTBEand0.0151to0.0351day–1forTBA.MTBEandTBAdegradationwereobservedonlyatsitesthatweremethano-genic(dissolvedmethane>0.5mg/L).Furthermore,thesesitesweredepletedinsulfaterelativetobackgroundconcentra-tions.Kolhatkarandothers(2002)confirmedtheanaerobicdegradationofMTBEandTBAinanoxicgroundwaterusingstablecarbonisotopeanalysis.ThisstudyalsoconcludesthattheanaerobicbiodegradationratesofMTBEandTBAmayexceedthoseestimatedforaerobicbiodegradation.

Anaerobic Biodegradation of Methyl Tert-butyl Ether

TheanaerobicbiodegradationofMTBEisknowntoproceed,althoughslowly,undermethanogenic(Wilsonandothers,2000;Wilsonandothers,2005),sulfate-reducing(Somsamakandothers,2001),iron-reducing(FinneranandLovley,2001),andnitrate-reducing(Bradleyandothers,2001)conditions.Littleispresentlyknown,however,aboutthepath-wayofMTBEbiodegradationunderanyoftheseconditions,althoughithasbeensuggestedthatanaerobicbiodegradationcouldbeinitiatedbyahydrolyticmechanism(O’Reillyandothers,2001;Kuderandothers,2005).


Table 1�. Average half-life for the aerobic biodegradation of the fuel compounds BTEX and methyl tert-butyl ether to carbon dioxide in an uncontaminated and contaminated matrix of aquifer sediments and ground water.1

[—,nostudiesreferenced;<,lessthan]

CompoundMedian1 primary degradation rate

(day – 1)

Average half-life in uncontaminated matrix (days)

Average half-life in contaminated matrix (days)

Field setting �

Laboratory

column �Laboratory microcosm

Field setting

Laboratory

column �In situ

microcosm

benzene 0.096 238 1.5 408 558 31 – 2.3 6,103 – 31

toluene .20 8135 – 238 4–7 8,940 – 60 75 2.3 64.5 – 7

ethylbenzene .113 238 — 960 – 139 — 2.3 13,1111

m-,p-xylene .054 238 — 8,931 – 60 — .350 12,113.5 – 11

o-xylene .054 238 1–4 912 – 25 — 2.3 6,1014 – 83

methyltert-butylether 12.0039 — — — — — 13<365

1Aronsonandothers,1999

2AmericanPetroleumInstitute,1994

3Alvarezandothers,1998

4Anidandothers,1993

5Kemblowskiandothers,1987

6Nielsenandothers,1996

7McCartyandothers,1998

8Barkerandothers,1987

9Hubbardandothers,1994

10Holmandothers,1992

11Thomasandothers,1990

12Laboratorymicrocosm

13Fennerandothers,2000

Table 18. Mean half-life in days for the anaerobic biodegradation of the fuel compounds BTEX, and methyl tert-butyl ether, tert-butyl alcohol under various reducing conditions.1

[(46),numberofsamplesusedtoderivethemeanvalue;MTBE,methyltert-butylether;TBA, tert-butylalcohol;—,notavailable]

Environmental condition Benzene Toluene Ethylbenzene o-Xylene m-Xylene p-Xylene MTBE TBA

Field/insitustudies 210(41) 12(46) 46(37) 33(33) 43(34) 46(26) — —

Nitrate-reducingstudies 97(38) 13(42) 104(28) 108(46) 113(35) 108(29) — —

Iron-reducingstudies 140(11) 516(10) 1,828(4) 1,822(8) 1,822(8) 1,822(8) — —

Sulfate-reducingstudies 50(9) 61(14) 197(7) 109(9) 141(8) 198(5) — —

Methanogenicstudies 61(16) 50(24) 229(8) 304(14) 317(10) 406(7) 2,330–7,302 2,315–5021AronsonandHoward,1997,p.16

2Kolhatkarandothers,2000

3Wilsonandothers,2005


benzene toluene(Azoarcus sp. strain T,Thauera aromatica )

o-xylene m-xylene p-xylene

phenol

benzyl-succinate

E-phenyl-itaconyl-

CoA

2-methyl-benzyl-

succinic acid

3-methyl-benzyl-

succinic acid

4-methyl-benzyl-

succinic acid

1-phenyl-ethyl-

succinic acid

1-phenyl-ethanol

4-phenyl-pentanoic

acid

acetophenone

2-methyl-phenyl-

itaconic acid

3-methyl-phenyl-

itaconic acid

4-methyl-phenyl-

itaconic acid

o-toluic acid m-toluic acid p-toluic acid

phthalicacid

isophthalicacid

terraphthalicacid

3-carboxybenzyl-succinic acid

Stable co-metabolic end members

o-toluic acid m-toluic acid p-toluic acid

benzoyl-CoA

benzoyl acetate

acetyl-CoA

(2)

(3)

ethylbenzene(unclassified Protobacteria

strain EB1)

benzoyl acetate Postulated compound from other experiments

Postulated microbe-catalyzed reductive pathway

(1) Beller and Spormann, 1997(2) Kniemeyer and Heider, 2001(3) Ball and others, 1996(4) Ulrich and others, 2005

(1)

(1)

(1)

benzylsuccinyl-CoA

(1)

(3)(3)

(4)

(4)

EXPLANATION

Literature reference describing reaction pathwayReductive pathway


Microorganism catalyzing reactionAzoarcus sp.

Figure 1�. Field and laboratory-derived pathways for the anaerobic biodegradation of the BTEX compounds — benzene, toluene, ethylbenzene, and xylene (modified from Edwards and Grbić-Galić,1994; and Griebler and others, 2004b).


methyl tert-butyl ether(Mycobacterium vaccae JOB5,

Mycobacterium austroafricanum IFP 2012)

hydroxymethyl tert-butyl ether

tert-butyl formate tert-butyl alcohol

2-methyl-2-hydroxy-1-propanol

2-hydroxyisobutyrate

2-propanolmethacrylate 2,3-dihydroxy-

2-methyl propionate

2-hydroxy-2-methyl-1,3-dicarbonate

l-lactate

carbon dioxide

formate

C1 metaboliccycle

methyl tert-butyl ether(Norcardia sp. ENV425)

hydroxymethyl tert-butyl ether

formaldehyde

C1 metaboliccycle

carbon dioxide

(1)

(1)

(1)

(1)

(1)

(2)

(2)

(2)

(2)(2) (2)

(2) (2)

(2)

(2)(1) Smith and others, 2003(2) Steffan and others, 1997

spontaneous reaction

EXPLANATION


Microorganism catalyzing reactionMycobacteriumvaccae


Hydrolysis

Hydration


Figure 1�. Laboratory-derived pathway for the aerobic biodegradation of methyl tert-butyl ether (modified from Pedersen and Essenberg, 2005).


(1) Keat and Hopper, 1978a(2) Keat and Hopper, 1978b(3) Hopper and Taylor, 1975(4) Poh and Bayly, 1980(5) Nakazawa and Hayashi, 1978

m-cresol (3-hydroxytoluene)(Pseudomonas alcaligenes NCIB 9867,

Pseudomonas putida NCIB 9869)

3-hydroxybenzyl alcohol

3-hydroxybenzaldehyde

3-hydroxybenzoate

3,4-dihydroxy-benzoate (Protocatechuate)

2,5-dihydroxy-benzoate(Gentisate)

2,4-dichlorobenzoate pathway

(1)

(2)

(3)

(4) (5)

toluene pathway(fig. 13)


Oxidation reaction

Hydroxylation reaction



EXPLANATION

Microorganism catalyzing reactionPseudomonasalcaligenes

Figure 18. Laboratory-derived pathway for the aerobic biodegradation of m-cresol (modified from Sakai and others, 2005).

styrene(Exophiala jeanselmei,

Pseudomonas putida CA-3)

styrene oxide

phenylacetaldehyde

phenylacetate

2-hydroxyphenylacetate

homogentisate

4-maleylacetoacetate

fumarylacetoacetate

fumarate acetoacetate

(1)

(1)

(1)

(2)

(2)

(3)

(4)

(5)

styrene(Rhodococcus rhodochrous

NCIMB 13259)

styrene cis-glycol

3-vinylcatechol

2-hydroxy-6-oxo-octa-trienoate

acrylate 2-hydroxypenta-2,4-dienoate


acetaldehyde pyruvate

(6)

(6)

(6)

(7)

(7)

(1) Cox and others, 1996(2) Olivera and others, 1994(3) Fernandez-Canon and Penalva,1995(4) Hagedorn and Chapman,1985(5) Crawford,1976(6) Warhurst and others, 1994(7) Lau and others, 1994

EXPLANATION


Microorganism catalyzing reactionExophialajeanselmei


Hydroxylation reaction

Hydrolysis

Hydration

Figure 19. Laboratory-derived pathways for the aerobic biodegradation of styrene (modified from Kraus and others, 2005).


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A

abiotic Notassociatedwithlivingorganisms.Synonymouswithabiological.1

abiotic transformation Processinwhichasubstanceintheenvironmentismodifiedbynonbiologicalmechanisms.1

absorption Thepenetrationofatoms,ions,ormoleculesintothebulkmassofasubstance.2

adsorption Theretentionofatoms,ions,ormoleculesontothesurfaceofanothersubstance.2

aerobe Anorganismthatneedsoxygenforrespirationandhenceforgrowth.1

aerobic Anenvironmentorprocessthatsustainsbiologicallifeandgrowth,oroccursonlywhenfree(molecular)oxygenispresent.2

aerobic conditions Conditionsforgrowthormetabolisminwhichtheorganismissufficientlysuppliedwithoxygen.1

alcohols Compoundsinwhichahydroxygroup,–OH,isattachedtoasaturatedcarbonatomR3COH.Thetermhydroxylreferstotheradicalspecies,HO.1

aldehydes CompoundsRC(=O)H,inwhichacarbonylgroupisbondedtoonehydrogenatomandtooneRgroup1.Rrepresentsafunctionalgroupsuchasanalkylgroup(methylorethylradical).1

aliphatic compounds Abroadcategoryofhydrocarboncompoundsdistinguishedbyastraight,orbranched,openchainarrangementoftheconstituentcarbonatoms,excludingaromaticcompounds.Thecarbon-carbonbondsmaybeeithersingleormultiplebonds.Alkanes,alkenes,andalkynesarealiphatichydrocarbons.2

alkanes Thehomologousgroupoflinear(acyclic)aliphatichydrocarbonshavingthegeneralformulaC

nH

2n+2.Alkanescan

bestraightchains,branchedchains,orringstructures,some-timescalledparaffins.1

alkenes Acyclicbranchedorunbranchedhydrocarbonshav-ingonecarbon–carbondoublebondandthegeneralformulaC

nH

2n,sometimescalledolefins.1

alkyl groups Univalentgroupsderivedfromalkanesbyremovalofahydrogenatomfromanycarbonatomwiththegeneralformof–C

nH

2n+1.Thegroupsderivedbyremovalof

ahydrogenatomfromaterminalcarbonatomofunbranchedalkanesformasubclasscallednormalalkyl(n-alkyl)groupsH[CH

2]

n.1

alkyl radicals Carbon-centeredradicalsderivedformallybyremovalofonehydrogenatomfromanalkane,e.g.,CH

3CH

2—(ethylradical).1

alkynes Thegroupofacyclicbranchedorunbranchedhydrocarbonshavingacarbon-carbontriplebondthathavethegeneralformulaC

nH

2n-2.1

ambient Thesurroundingenvironmentandprevailingconditions.2

anaerobe Anorganismthatdoesnotneedfree-formoxygenforgrowth.Manyanaerobesareevensensitivetofreeoxygen.1

anaerobic Abiologically-mediatedprocessorconditionnotrequiringmolecularorfreeoxygen.1

analyte Thecomponentofasystemtobeanalyzed.1Forexample,chemicalelementsorionsinground-watersample.2

anoxic Anenvironmentwithoutoxygen.2

aquifer Awater-bearinglayerofsoil,sand,gravel,rockorothergeologicformationthatwillyieldusablequantitiesofwatertoawellundernormalhydraulicgradientsorbypumpage.3

aromatic Agroupoforganiccompoundsthatarecyclic,containresonantcarbon-carbondoublebondsintheformofatleastone6-carbonbenzenering.2Inthetraditionalsense,“havingachemistrytypifiedbybenzene.”1

attenuation Thesetofhuman-madeornaturalprocessesthateitherreduceorappeartoreducetheamountofachemi-calcompoundasitmigratesawayfromonespecificpointtowardsanotherpointinspaceortime.Forexample,theapparentreductionintheamountofachemicalinaground-waterplumeasitmigratesawayfromitssource.Degradation,dilution,dispersion,sorption,orvolatilizationarecommonprocessesofattenuation.2

B

biodegradation Transformationofsubstancesintonewcompoundsthroughbiochemicalreactionsortheactionsofmicroorganisms,suchasbacteria.Typicallyexpressedintermsofarateconstantorhalf-life.2

biota Livingorganisms.2

breakdown product Acompoundderivedbychemical,biological,orphysicalactiononachemicalcompound.Thebreakdownisaprocesswhichmayresultinamoretoxicoralesstoxiccompoundandamorepersistentorlesspersistentcompoundthantheoriginalcompound.2

GlossarySources:1InternationalUnionofPureandAppliedChemistry,2006;2U.S.EnvironmentalProtectionAgency,2004;3Wiedemeierandothers,1998;4U.S.GeologicalSurvey,2006;5BrownandLeMay,1977


C

carbon Elementnumber6intheperiodictableofelements.Foradescriptionofthevarioustypesofcarbonasasolid,thetermcarbonshouldbeusedonlyincombinationwithanaddi-tionalnounoraclarifyingadjective(thatis,organiccarbon).1

catabolism Thebreakdownofcomplexmoleculesintosimpleronesthroughtheoxidationoforganicsubstratestoprovidebiologicallyavailableenergy(forexample,ATP,adenosinetriphosphate).1

catalysis TheprocesswhereacatalystincreasestherateofachemicalreactionwithoutmodifyingtheoverallstandardGibbsenergychangeinthereaction.1

catalyst Substancesthatincreasestherateofachemi-calreaction.Thecatalystisbothareactantandproductofthereaction.Thewordscatalystandcatalysisshouldnotbeusedwhentheaddedsubstancereducestherateofreaction(seeinhibitor).1

chemical bond Theforcesactingamongtwoatomsorgroupsofatomsthatleadtotheformationofanaggregatewithsufficientstabilitytomakeitconvenientforthechemisttoconsideritasanindependent“molecularspecies.”1

chemical induction (coupling) Whenonereactionacceler-atesanotherinachemicalsystemthereissaidtobechemicalinductionorcoupling.Couplingiscausedbyanintermedi-ateorby-productoftheinducingreactionthatparticipatesinasecondreaction.Chemicalinductionisoftenobservedinoxidation–reductionreactions.1

chemical reaction Aprocessthatresultsintheinterconver-sionofchemicalspecies.Chemicalreactionsmaybeelemen-taryreactionsorstepwisereactions.1

chlorinated solvent Avolatileorganiccompoundcontain-ingchlorine.Somecommonsolventsaretrichloroethylene,tetrachloroethylene,andcarbontetrachloride.2

cis, trans isomers Thedifferenceinthepositionsofatoms(orgroupsofatoms)relativetoareferenceplaneinanorganicmolecule.Inacis-isomer,theatomsareonthesamesideofthemolecule,butareonoppositesidesinthetrans-isomer.Sometimescalledstereoisomers,thesearrangementsarecom-moninalkenesandcycloalkanes.1

co-metabolism Thesimultaneousmetabolismoftwocom-pounds,inwhichthedegradationofthesecondcompound(thesecondarysubstrate)dependsonthepresenceofthefirstcompound(theprimarysubstrate).Forexample,intheprocessofdegradingmethane,somebacteriacandegradechlorinatedsolventsthatwouldotherwisenotbedegradedunderthesameconditions.2

concentration Compositionofamixturecharacterizedintermsofmass,amount,volumeornumberconcentrationwithrespecttothevolumeofthemixture.1

conservative constituent or compound Onethatdoesnotdegrade,isunreactive,anditsmovementisnotretardedwithin

agivenenvironment(aquifer,stream,contaminantplume,andsoforth).4

constituent Anessentialpartorcomponentofasystemorgroup(thatis,aningredientofachemicalmixture).Forinstance,benzeneisoneconstituentofgasoline.4

covalent bond Aregionofrelativelyhighelectrondensitybetweenatomicnucleithatresultsfromsharingofelectronsandthatgivesrisetoanattractiveforceandacharacteristicinternucleardistance.Carbon-hydrogenbondsarecovalentbonds.1

D

daughter product Acompoundthatresultsdirectlyfromthedegradationofanother.Forexamplecis-1,2-dichloroethene(12-cDCE)iscommonlyadaughterproductoftrichloroethene(TCE)degradation.Thisisatermthathascurrently(2006)fallenoutofgeneraluse.Seemetabolicby-product.3

dehydrohalogenation Removalofhydrogenandhalideionsfromanalkaneresultingintheformationofanalkene.3

denitrification Bacterialreductionofnitratetonitritetogas-eousnitrogenornitrousoxidesunderanaerobicconditions.4

density (ρ) Theratioofthemassofasubstancetothemassofanequalvolumeofdistilledwaterat4degreesCelsius.Sincethemassofonemilliliter(ml)ofwaterat4degreesCelsiusisexactly1gram,thespecificgravity(unitless)isnumericallyequivalenttoitsdensity(ingramsperml).1

detection limit (in analysis) Theminimumsingleresultthat,withastatedprobability,canbedistinguishedfromarepresen-tativeblankvalueduringthelaboratoryanalysisofsubstancessuchaswater,soil,air,rock,biota,tissue,blood,andsoforth.1

dichloroelimination Removaloftwochlorineatomsfromanalkanecompoundandtheformationofanalkenecompoundwithinareducingenvironment.4

dihaloelimination Removaloftwohalideatomsfromanalkanecompoundandtheformationofanalkenecompoundwithinareducingenvironment.3

diols Chemicalcompoundsthatcontaintwohydroxy(--OH)groups,generallyassumedtobe,butnotnecessarily,alcoholic.Aliphaticdiolsarealsocalledglycols.1

downgradient Inthedirectionofdecreasingstatichydraulichead(potential).4

E

electron acceptor Acompoundcapableofacceptingelec-tronsduringoxidation-reductionreactions.Microorganismsobtainenergybytransferringelectronsfromelectrondonorssuchasorganiccompounds(orsometimesreducedinorganiccompoundssuchassulfide)toanelectronacceptor.Electronacceptorsarecompoundsthatarerelativelyoxidizedandincludeoxygen,nitrate,iron(III),manganese(IV),sulfate,carbondioxide,orinsomecaseschlorinatedaliphatichydro-


carbonssuchasperchloroethene(PCE),TCE,DCE,andvinylchloride.2

electron donor Acompoundcapableofsupplying(givingup)electronsduringoxidation-reductionreactions.Microor-ganismsobtainenergybytransferringelectronsfromelectrondonorssuchasorganiccompounds(orsometimesreducedinorganiccompoundssuchassulfide)toanelectronacceptor.Electrondonorsarecompoundsthatarerelativelyreducedandincludefuelhydrocarbonsandnativeorganiccarbon.3

electronegativity ConceptintroducedbyNobelLaureateLinusPaulingasthepowerofanatomtoattractelectronstoitself.1

elimination Reactionwheretwogroupssuchaschlorineandhydrogenarelostfromadjacentcarbonatomsandadoublebondisformedintheirplace.3

endergonic reaction Achemicalreactionthatrequiresenergytoproceed.Achemicalreactionisendergonicwhenthechangeinfreeenergyispositive.3

enzyme Macromolecules,mostlyproteinsorconjugatedpro-teinsproducedbylivingorganisms,thatfacilitatethedegrada-tionofachemicalcompound(catalyst).Ingeneral,anenzymecatalyzesonlyonereactiontype(reactionspecificity)andoperatesononlyonetypeofsubstrate(substratespecificity).1,4

epoxidation Areactionwhereinanoxygenmoleculeisinsertedinacarbon-carbondoublebondandanepoxideisformed.3

epoxides Asubclassofepoxycompoundscontainingasatu-ratedthree-memberedcyclicether.Seeepoxycompounds.1

epoxy compounds Compoundsinwhichanoxygenatomisdirectlyattachedtotwoadjacentornonadjacentcarbonatomsinacarbonchainorringsystem;thuscyclicethers.1

F

facultative anaerobes Microorganismsthatuse(andprefer)oxygenwhenitisavailable,butcanalsousealternateelectronacceptorssuchasnitrateunderanaerobicconditionswhennecessary.3

fermentation Microbialmetabolisminwhichaparticularcompoundisusedbothasanelectrondonorandanelectronacceptorresultingintheproductionofoxidizedandreduceddaughterproducts.3

functional group Anatom,oragroupofatomsattachedtothebasestructureofacompoundthathassimilarchemicalpropertiesirrespectiveofthecompoundtowhichitisapart.Itdefinesthecharacteristicphysicalandchemicalpropertiesoffamiliesoforganiccompounds.1

G

H

half-life (t½ ) Thetimerequiredtoreducetheconcentrationofachemicalto50percentofitsinitialconcentration.Unitsaretypicallyinhoursordays.2

halide Anelementfromthehalogengroup.Theseincludefluorine,chlorine,bromine,iodine,andastatine.5

halogen Group17intheperiodictableoftheelements.Theseelementsarethereactivenonmetalsandareelectronegative.5

Henry’s Law TherelationbetweenthepartialpressureofacompoundandtheequilibriumconcentrationintheliquidthroughaproportionalityconstantknownastheHenry’sLawconstant.4

Henry’s Law constant Theconcentrationratiobetweenacompoundinair(orvapor)andtheconcentrationofthecom-poundinwaterunderequilibriumconditions.4

heterogeneous Varyinginstructureorcompositionatdiffer-entlocationsinspace.4

heterotrophic Organismsthatderivecarbonfromorganicmatterforcellgrowth.4

homogeneous Havinguniformstructureorcompositionatalllocationsinspace.4

hydration Theadditionofawatermoleculetoacompoundwithinanaerobicdegradationpathway.5

hydrogen bond Aformofassociationbetweenanelectro-negativeatomandahydrogenatomattachedtoasecond,relativelyelectronegativeatom.Itisbestconsideredasanelectrostaticinteraction,heightenedbythesmallsizeofhydro-gen,whichpermitscloseproximityoftheinteractingdipolesorcharges.1

hydrogenation Aprocesswherebyanenzymeincertainmicroorganismscatalyzesthehydrolysisorreductionofasubstratebymolecularhydrogen.2

hydrogenolysis Areductivereactioninwhichacarbon-halogenbondisbroken,andhydrogenreplacesthehalogensubstituent.3

hydrolysis Achemicaltransformationprocessinwhichachemicalreactswithwater.Intheprocess,anewcarbon-oxygenbondisformedwithoxygenderivedfromthewatermolecule,andabondiscleavedwithinthechemicalbetweencarbonandsomefunctionalgroup.1

hydroxylation Additionofahydroxylgrouptoachlorinatedaliphatichydrocarbon.3

I

inhibition Thedecreaseinrateofreactionbroughtaboutbytheadditionofasubstance(inhibitor),byvirtueofitseffectontheconcentrationofareactant,catalyst,orreactionintermediate.1

in situ Initsoriginalplace;unmoved;unexcavated;remain-inginthesubsurface.4

J

K


L

lag phase Thegrowthinterval(adaptionphase)betweenmicrobialinoculationandthestartoftheexponentialgrowthphaseduringwhichthereislittleornomicrobialgrowth.1

M

measurement Adescriptionofapropertyofasystembymeansofasetofspecifiedrules,thatmapsthepropertyontoascaleofspecifiedvalues,bydirector“mathematical”comparisonwithspecifiedreference(s).1

metabolic by-product (by-product) Aproductofthereactionbetweenanelectrondonorandanelectronacceptor.Metabolicby-productsincludevolatilefattyacids,daughterproductsofchlorinatedaliphatichydrocarbons,methane,andchloride.3

metabolism Theentirephysicalandchemicalprocessesinvolvedinthemaintenanceandreproductionoflifeinwhichnutrientsareusedtogenerateenergyandintheprocessde-gradetosimplermolecules(catabolism),whichbythemselvesmaybeusedtoformmorecomplexmolecules(anabolism).1

methanogens Strictlyanaerobicarchaebacteria,abletouseonlyaverylimitedspectrumofsubstrates(forexample,molecularhydrogen,formate,methanol,methylamine,carbonmonoxideoracetate)aselectrondonorsforthereductionofcarbondioxidetomethane.1

methanogenic Theformationofmethanebycertainanaerobicbacteria(methanogens)duringtheprocessofanaerobicfermentation.4

microcosm Adiminutive,representativesystemanalogoustoalargersystemincomposition,development,orconfiguration.4

microorganisms Microscopicorganismsthatincludebacte-ria,protozoans,yeast,fungi,mold,viruses,andalgae.4

mineralization Thereleaseofinorganicchemicalsfromorganicmatterintheprocessofaerobicoranaerobicdecay.4

monoaromatic Aromatichydrocarbonscontainingasinglebenzenering.4

N

nucleophile Achemicalreagentthatreactsbyformingcova-lentbondswithelectronegativeatomsandcompounds.4

nutrients Majorelements(forexample,nitrogenandphosphorus)andtraceelements(includingsulfur,potassium,calcium,andmagnesium)thatareessentialforthegrowthoforganisms.3

O

octanol-water partition coefficient (KOW) Theequilibriumratioofachemical’sconcentrationinoctanol(analcoholiccompound)toitsconcentrationintheaqueousphaseofatwo-phaseoctanol/watersystem,typicallyexpressedinlogunits(logK

OW).K

OWprovidesanindicationofachemical’s

solubilityinfats(lipophilicity),itstendencytobioconcentrateinaquaticorganisms,orsorbtosoilorsediment.2

order of reaction Achemicalrateprocessoccurringinsystemsforwhichconcentrationchanges(andhencetherateofreaction)arenotthemselvesmeasurable,provideditispossibletomeasureachemicalflux.1

organic carbon (soil) partition coefficient (KOC) Thepropor-tionofachemicalsorbedtothesolidphase,atequilibriuminatwo-phase,water/soilorwater/sedimentsystemexpressedonanorganiccarbonbasis.ChemicalswithhigherK

OCvaluesare

morestronglysorbedtoorganiccarbonand,therefore,tendtobelessmobileintheenvironment.2

oxidation Ingeneral,areactioninwhichelectronsaretransferredfromachemicaltoanoxidizingagent,orwhereachemicalgainsoxygenfromanoxidizingagent.2

P

Q

R

rate Derivedquantityinwhichtimeisadenominatorquantity.Rateofxisdx/dt.1

rate constant, k Seeorderofreaction.1

rate-controlling step (rate-limiting step, rate-determining step) Theelementaryreactionhavingthelargestcontrolfac-torexertsthestrongestinfluenceontherate(v).Astephavingacontrolfactormuchlargerthananyotherstepissaidtoberate-controlling.1

recalcitrant Unreactive,nondegradable,refractory.4

redox Reduction-oxidationreactions.Oxidationandreduc-tionoccursimultaneously;ingeneral,theoxidizingagentgainselectronsintheprocess(andisreduced)whilethereduc-ingagentdonateselectrons(andisoxidized).2

reduction Ingeneral,areactioninwhichelectronsaretrans-ferredtoachemicalfromareducingagent,orwhereoxygenisremovedfromachemical.2

respiration Theprocessofcouplingoxidationoforganiccompoundswiththereductionofinorganiccompounds,suchasoxygen,nitrate,iron(III),manganese(IV),andsulfate.2

S

solvolysis Generally,areactionwithasolvent,involvingtheruptureofoneormorebondsinthereactingsolute.Morespecificallythetermisusedforsubstitution,elimination,orfragmentationreactionsinwhichasolventspeciesisthenucleophile(hydrolysis,ifthesolventiswateroralcoholysis,ifthesolventisanalcohol).1

stable Asappliedtochemicalspecies,thetermexpressesathermodynamicproperty,whichisquantitativelymeasuredbyrelativemolarstandardGibbsenergies.AchemicalspeciesAismorestablethanitsisomerBif∆rGo>0forthe(realorhypothetical)reactionA→ B,understandardconditions.1


substrate Componentinanutrientmedium,supplyingmicroorganismswithcarbon(C-substrate),nitrogen(N-sub-strate)as“food”neededtogrow.1

T

terminal electron acceptor (TEA) Acompoundormoleculethatacceptsanelectron(isreduced)duringmetabolism(oxi-dation)ofacarbonsource.Underaerobicconditionsmolecu-laroxygenistheterminalelectronacceptor.Underanaerobicconditionsavarietyofterminalelectronacceptorsmaybeused.Inorderofdecreasingredoxpotential,theseTEAsincludenitrate,manganicmanganese,ferriciron,sulfate,andcarbondioxide.Microorganismspreferentiallyutilizeelec-tronacceptorsthatprovidethemaximumfreeenergyduringrespiration.Ofthecommonterminalelectronacceptorslistedabove,oxygenhasthehighestredoxpotentialandprovidesthemostfreeenergyduringelectrontransfer.4

U

unsaturated zone Thezonebetweenlandsurfaceandthecapillaryfringewithinwhichthemoisturecontentislessthansaturationandpressureislessthanatmospheric.Soilporespacesalsotypicallycontainairorothergases.Thecapillaryfringeisnotincludedintheunsaturatedzone.4

upgradient Inthedirectionofincreasingpotentiometric(piezometric)head.4

V

vadose zone Thezonebetweenlandsurfaceandthewatertablewithinwhichthemoisturecontentislessthansaturation(exceptinthecapillaryfringe)andpressureislessthanatmo-spheric.Soilporespacesalsotypicallycontainairorothergases.Thecapillaryfringeisincludedinthevadosezone.4

vapor pressure (Pv) Theforceperunitareaexertedbyavaporinanequilibriumstatewithitspuresolid,liquid,orsolutionatagiventemperature.Vaporpressureisameasureofasubstance’spropensitytoevaporate.Vaporpressureincreasesexponentiallywithanincreaseintemperature.Typicalunitsaremillimeterofmercury(mmHg),torr,orinchesofmercury(in.Hg).2

W

water solubility (S) Themaximumamountofachemicalthatcanbedissolvedinagivenamountofpurewateratstandardconditionsoftemperatureandpressure.Typicalunitsaremil-ligramsperliter(mg/L),gallonsperliter(g/L),orpoundspergallon(lbs/gal).2

X

Y

Z


Manuscript approved for publication, October 23, 2006Prepared by USGS Georgia Water Science CenterEdited by Patricia L. NoblesGraphics by Caryl J. WipperfurthFor more information concerning the research in this report, contact

USGS Georgia Water Science Center, Atlanta, telephone: 770-903-9100

description, properties, and degradation of selected volatile

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