al

Robert D. Morrison*

Es

Au

tinityntaicur goengn s

degni

and contaminant transport modeling. Part I of thisseries discusses a review of aerial photographyintecalsmedatuseateanachlbipstrathe

2000d) and in conference proceedings (University of

Petroleum Hydrocarbons

etc.) (Zemo, Bruya and Graf, 1995). If sucient spill

single event, a series of events, or a continuous releaseof a single or multiple products. This determination isaccomplished through pattern recognition or matching

m-dend

ier

nd

es

20 30 30 id

. crude oils and heavy fuels (substituted polyaromatic

estimates regarding the age of the petroleum hydro-carbon, or to assign a range of years (i.e. within a

Environmental Forensics (2000) 1, 175195

doi:10.1006/enfo.2000.0018, available online at http://www.idealiForensic techniques available for age dating and sourceidentification of petroleum hydrocarbons include

10-year window, etc.), that is based upon the degree ofsample weathering. Frequent challenges to this argu-Wisconsin, 2000; International Business Communica-tions, 2000).

hydrocarbons, and thiophenes).

The use of chromatographic pattern recognition isfrequently argued as a means to develop qualitativeBruya and Graf, 1995; Kaplan and Galperin, 1996;Kaplan, Galperin and Lee, 1997; Morrison, 1999a,c;

compounds (i.e., present in some petroleum productsbut not creosote) (Butler, 1999);hydadd

1527

*E-mrpretation, corrosion models, associations of chemi-with specific manufacturing activities or equip-

nt, and chlorinated solvent applications for ageing and source identification. Part II discusses theof petroleum hydrocarbon age dating and associ-d source identification techniques, stable isotopelysis and contaminant transport modeling usingorinated solvents (especially TCE), polychlorinatedhenyls (PCBs), and petroleum products to demon-te the applicability of the methods. Discussions ofse techniques are included in the literature (Zemo,

of gas chromatogram traces (Rohrbach, 1998). Copounds used in pattern recognition analysis inclu(Harvey, 1997; Stout, 1999; Stout, Uhler aMcCarthy, 1999; Stout et al., 1999a,b):

. light petroleum products (BTEX and heavaromatics, alkylate parans);

. diesel fuels and distillates (normal alkanes aisoprenoid parans);

. biomarkers such as sesquiterpanes (C15), diterpan(C ), triterpanes (C ), steranes (C , and hopanoR. Morrison & Associates Inc., 201 East Grand Avenue,

(Received 28 April 2000, Revised manuscript accepted 1

A multitude of forensic techniques are available for age dainterpretation, corrosion models, the commercial availabilequipment, chemical profiling, degradation models and coin environmental litigation and their applicability to a partthese techniques are introduced as scientific evidence, theidata are rigorously scrutinized and often, successfully challtechniques and discuss their merits so that the user caappropriate for the factual elements of the case.

Keywords: degradation model; reverse groundwater mochemical fingerprinting; chemical pattern reco

Introduction

Identification of the origin of a contaminant release, thetiming of the release, and its distribution in thesubsurface are common issues in environmental litiga-tion. Forensic techniques used to investigate theseissues include, in part, aerial photography interpret-ation, underground storage tank corrosion models,identifying the date when a chemical or additive becamecommercially available, association of a particularchemical with a manufacturing process, chemical pro-filing (fingerprinting), chemical degradation models,Critical Review of Environmentrocarbon pattern recognition, use of proprietaryitives, alkyl-lead speciation, oxygenates, dyes, stable

175

-5922/00/040175+20 $35.00/00

ail: bob@rmorrison.comrelease information is available, this technique canoften discriminate whether a chemical release was aForensic Techniques: Part II

condido, CA 92025, U.S.A.

gust 2000)

g and source identification, including aerial photographyof a chemical, chemical associations with discrete types ofminant transport models. The success of these techniqueslar fact situation is rarely discussed in the literature. Whenverning assumptions and the adequacy of the underlyinged. The purpose of this paper is to review selected forensicelect the technique or combination of techniques most

# 2000 AEHS

ling; backward extrapolation modeling; inverse model;tion; proprietary additives; alkyl-lead.

isotope analyses, weathering patterns, biomarkers, anddegradation models.

Fingerprinting

Petroleum hydrocarbon fingerprinting or patternrecognition allows identification of discrete fuel types(i.e. diesel, gasoline, jet fuels, marine fuel, kerosene,

brary.com onment include that there are substantial dierences inweathering which are attributable to the subsurfaceenvironment (anaerobic v. aerobic transformation) andsoil texture (e.g. moist clay v. moist sand). Additionally,

# 2000 AEHS

there ahydrocarelease.tool whfor compatternsame enobservawhen unot for

Propri

Propriesuch asdiscreteblendedpresent1982).printingpackagmaskedmentaltion ofadditiveand aredegradeSome ptheir rethe parsold tothe refipresentthose fwater (Ethyl1999a,bfor refinfor auto62% teethylenas dyesimprovblendeddiesel inand nitanti-oxlkylphe(ethylenchloridcarbons), metal deactivators (N,N-disalicylidene-alkyl-diamines), surfactants (alcohols, amines, alkylphenols,carboxybustionbariumene glyfuels toisms (C1997).inhibitoammon(N,N-dcyliden1,2-etha

-di).sueoleutifiwict mentiplagletio

yl-

nor arongeical-lelab; Mdaselveusteadbinyl-tionistrethethyltcted25,mouretioneth, 2yl-hylnreyl-0,trampentSchmidt, 1994). Older gasolines contain tetraethyl-

lead anethylen

waenhe pede thpos syl-vidn

176 R.lic acids, sulfonates, succinamides) and com-catalysts/deposit modifiers (organometallics of, calcium magnesium, iron, manganese). Ethyl-col monomethyl ether, is present in some dieselprevent clogging of fuel lines by microorgan-entral Regional Water Quality Control Board,The jet fuel, JP-4, for example, contains icingrs (carboxylates, alcohols, dimethylformamide,ium dinonylnaphthalene), metal deactivatorsisalicylidene-1,2-propanediamine, N,N-disali-

leadconcT

arguwhilof aleaddiethas eChinre uncertainties associated with the degree ofrbon weathering that occurred prior to aChromatographic age dating may be a usefulen there are chromatographic patterns availableparison, and when those chromatograms ares of a similar product that was released into thevironment at a known time. It is the authorstion that pattern recognition is most usefulsed as a comparative tool between samples, andage dating.

etary Additives

tary additives are blended with refined products,fuels. Additives are frequently associated withtime periods corresponding to when they werewith a product (i.e. polybutene additive wasin the Chevron detergent F-310 in gasoline inThe use of additives for hydrocarbon finger-requires a prior knowledge of the additive

e, and the ability to detect an additive that is notby other chemicals or obscured by environ-

degradation. In practice, the chemical identifica-an additive is not always straightforward. Manys contain oxygen in their molecular structurehighly soluble and/or biodegradable; they mayand/or transform rapidly in the subsurface.

olymers, for example, rapidly depolymerize intospective monomers making it dicult to identifyent compound (Galperin, 1997). Additives arerefineries with little or no chemical alteration bynery. As a result, the same additive may bein the parent compounds of multiple fuels anduels may become commingled in the ground-Kram, 1988; Gibbs, 1990, 1993; Harvey, 1997;Corporation, 1998; Morrison, 1998a,b,,c,d). The composition of additive packagesed products varies with time. A typical mixturemotive gasoline in the 1980s consists of abouttraethyl-lead, 18% ethylene dibromide, 18%e dichloride, and 2% inactive ingredients such, antioxidants, petroleum solvents, and stabilityers (Kaplan et al., 1997). Additives are alsowith diesel and jet fuels. Additive packages forclude diesel ignition compounds (alkyl nitratesrites, nitro-, nitroso-compounds and peroxides),idants (N,N-dialkylphenylenediamines, 2,6-dia-nols, 2,4,6-trialkylphenols), cold flow improverse vinyl acetate copolymers, ethylene vinyle copolymers, polyolefins, chlorinated hydro-

N,N1998Is

petridenbersdetepresmultpackknowaddi

Alk

Chrootheenvifor apredalkyavai1998addeincrea vaexhaL

comdiethreacRedtetratrimmethReaRMthemixtreactetraleaddiethtrietof umeth20 :8Te

to ipresand

D. Morrisone-1,2-cyclohexanediamine, N,N-disalicylidene-nediamine) and anti-oxidants (alkyphenols,

tion ofbeing td the lead scavengers ethylene dibromide ande dichloride (1,2-dichloroethane). Tetraethyl-s blended with gasoline prior to about 1985 attrations of about 400 to 500 mg/L.resence of organic lead in gasoline is frequentlyas evidence of a pre-1985/86 gasoline release,e presence of only tetraethyl-lead is indicativest-1980 release. The presence of non-tetraethyluch as methyldiethyl-, tetramethyl-, dimethyl-, tetramethyl- or trimethyl-lead is then arguedence of a pre-1980 release. Hurst, Davis and(1996), for example, reported that The specia--sec-butyl-p-phenylendiamine) (Potter et al.,

s raised frequently when relying uponm additives for age dating and/or sourcecation include product swapping between job-th dierent additive packages, the inability toost additives unless phase separate-product is

, commingling of fuels in the subsurface,e releases of product with dierent additivees at the same location, and an incompletedge of the original additive package. Inn, many fuel additives are proprietary.

leads

logies based on the blending of alkyl-leads withdditives into fuel products and their presence inmental samples is frequently cited as evidencedating a product release. This approach is oftented on the documented changes in the use ofads and/or knowledge of the composition andility of additive packages (Gibbs, 1990; Harvey,orrison, 2000a,b,c). Alkyl-leads were initiallyto gasoline to suppress spark knock and tothe octane number, although they also serve aslubricant by forming a protective coating on thevalve seat (EPA, 1999).additive packages often contain multiple

ations of tetraethyl-, triethylmethyl-, methyl-and tetramethyl-leads, as well as redistributionmixtures of tetraethyl- and trimethyl-leads.

ibution reactions of equimolar amounts ofyl- and tetramethyl-leads can also produceyl-, trimethylethyl-, dimethyldiethyl- andriethyl-leads (Christensen and Larsen, 1993).mixtures of leads are typically marketed as

RM50, and RM75 with the number designatinglar percent of trimethyl-lead present in the(Stout et al., 1999b). A typical commercialmixture from the use of equimolar amounts ofyl-lead and trimethyl-lead is 3.8% trimethyl-3.4% trimethylethyl-lead, 42.4% dimethyl-lead, 25.6% methyltriethyl-lead, and 4.8%-lead (Kaplan et al., 1997). Physical mixturesacted combinations of tetraethyl- and tetra-leads are described in percentages, such as50 :50, 80 :20, etc.ethyl-lead is used to suppress pre-ignition androve the octane rating of the fuel. It is notin condensates, distillates, or naphtha (Brucelead additives has changed, with tetraethyl leadhe only alkyl-lead additive in leaded fuels after

of manganese-based substances, including MMT. In1998, this law was abolished and MMT is currently anadditive in Canadian gasoline. Given this hiatus inMMT usage, it is dicult to use MMT as an agediagnostic technique beyond its introduction in Canadaprior to 1976.The lead scavengers ethylene dibromide (EDB) and

ethylene dichloride (EDC) were introduced in19271928 to alleviate problems with metal corrosionin engines caused by the formation of lead oxide in thecombustion chamber that resulted in damaged sparkplugs and exhaust valves (Kaplan et al., 1997). EDBand EDC minimized the precipitation of lead oxides inthe engines by forming the relatively volatile lead

Review of Environmental Forensic Techniques 1771980. Hence, analyses of alkyl-lead species present infree products can help date the time of a release.Other literature, suggests that the phasing out oftetraethyl-lead continued into the early to late 1980s(Kaplan, 2000a; Peterson, 2000; Clark, no date). Careis therefore required when extrapolating generalizedalkyl-lead chronologies. Additional complicationsinclude the re-mobilization and dissolution of tetra-ethyl-lead by subsequent gasoline releases with orwithout additive packages migrating through the samesoil. The tested sample may therefore be a composite ofseveral releases containing multiple additive packages.Regulatory changes in the allowable alkyl-lead

concentrations in gasoline in conjunction with leadconcentration data are frequently forwarded as evid-ence for age dating a release. In 1982, the maximumlead concentration in gasoline was 4.2 g/gallon. In 1984,the Environment Protection Agency set a maximum of0.1 g/gallon. This concentration applies to the averagequarterly production from a refinery or a poolstandard. The pool standard is the total grams of leadused by a refinery in a given time period divided by thetotal amount of gasoline manufactured in the same timeframe. A typical challenge to this argument includesuncertainties associated with the original lead content.An example of this argument is that lead results of anindividual sample are not conclusive because the leadcontent for any point in time is based on the pool stan-dard. The consequence of this practice is that individualgasoline samples vary from batch to batch and cannotbe used to date the year of manufacture. The poolstandard may not reflect any true refinery amount dueto lead accounting practices (leads credits are bought orsold) that are usually averaged quarterly. In addition,multiple releases of gasoline with low lead concen-trations from 1985 to 1991 can result in an accumulatedlead concentration of 0.5 g/gallon that can be mis-interpreted as evidence of a pre-1985 gasoline release.Non-lead alkyl additives in fuels with alkyl-leads

may provide additional forensic evidence by providingresolution for age dating the product. Examplesinclude nickel carbonyl, dicyclopentadienyl-iron, ironpentacarbonyl, methylcyclopentadienyl manganese tri-carbonyl (MMT), and lead scavengers. MMT was usedas an octane improver in unleaded gasoline (Zayed,Hong and LEsperance, 1999). MMT synonyms andtrade names include CL-2, Combustion Improver-2,manganese tricarbonyl-methylcyclopentadientyl, and2-methylcyclopentadientyl. MMT was introduced asan antiknock and alkyl-lead supplement in the UnitedStates in about 1958/59 by Ethyl Corporation asAK-33X, and was available until about 1977/78(Harvey, 1998; Bruya, 2000; Peterson, 2000) althougha few refineries used it until 1985 (Kaplan, 2000b).MMT was also blended with tetraethyl-lead as asupplement (Gibbs, 1990; Hurst, Davis and Chinn,1996; Ethyl Corporation, 1998).While the detection of MMT may be age diagnostic,

it was not routinely added by all manufacturers and hasdierent usage histories throughout the world. MMTwas blended in gasoline in Canada, for example, since1976. MMT usage increased substantially when itreplaced lead in gasoline in 1990 (Zayed, Hong and

LEsperance, 1997). In 1997, Canada adopted law C-29that banned the inter-provincial trade and importationbromide, or lead chloride, that passed through theengine with the exhaust. The concentration of EDB andEDC in leaded gasoline has changed over the years. Thequantity of EDB and/or EDC in a lead package isdetermined by the amount of alkyl-lead present. Asucient amount of scavenger is added to theoreticallyreact with all the lead, which is termed one theory, with1.0 to 1.5 theories typically used. EDB is currentlyadded to fuels used in aviation piston engines (CentralRegional Water Quality Control Board, 1997).EDB and EDC are moderately soluble (4.32 and

8.69 g/L at 208C, respectively). Given that alkyl-leadsare strongly adsorbed to soil and tend to be hydro-lyzed, the presence of lead scavengers may be the onlyresidue of a leaded gasoline release. For phase-separateleaded gasoline in contact with groundwater, thecontact time between the two fluids and the resultingscavenger dissolution into the groundwater has beenused for age dating. Challenges to this argumentinclude the potential for the re-dissolution of alkyl-leads or lead scavengers into non-leaded gasolinemigrating through the location of an earlier release,and fluctuating EDB and EDC concentrations due to arising and falling groundwater that is in contact withthe gasoline-impacted soil (Morrison, 2000b).The distribution and ratios of alkyl-leads and lead

scavengers in phase-separate samples can be used toidentify the locations of multiple releases. Figure 1depicts the ratio of total alkyl-lead and EDB in phase-separate gasoline in monitoring wells located down-gradient of a documented release (Morrison, 2000c).The decrease in the total alkyl-lead/EDB ratio v. thedistance from the spill, to a distance of about 500 to600 feet, is consistent with a single documented release.The abrupt increase in the total alkyl-lead/EDB ratio

0 200 400 600 800 1000

100

10

1

Distance from documented release (feet)

Tot

al a

lkyl

lead

/ED

B r

atio

Documented release Suspected release location

Figure 1. Total alkyl lead/EDB ratio as a function from a known and

suspected leaded gasoline release from monitoring wells containingphase-separate gasoline (after Morrison, 2000a).

at a diswith asubsequsecondChal

ility thaconcenface wrathercation min the afunctiogasolinwashingseveralgasolinEDC anof prodtotal ainsightIn oldeare stroupon colead sca

Oxyge

Oxygenoxygensions. Tdefinesashless,ether, w(Gibbs,blendedits widepurposincreaseTertiary-butyl alcohol (TBA) was available in 1969 andwas blenol blenof alcohoxygenhol (IPethyl teether (T(CentraAtlan

methylcatalytimethan1998).aboutSteanof MTBBy 1999the Unwere aldomestand CitUnitedMTB

replacewas latamoun

to, 2imdi; Deveble neonsEEwesst g; Dusraleninto

darncye GrisTB2,0asuenr plbevioe lTBBy anda

ively. Atraennchetioes amp

tenstinrioaversttenidcidsololodprosspecerTB

178 R.nded with methanol in 1981, although metha-ds are no longer used (Gibbs, 1998). Examplesols and ethers used as octane enhancers and/orates include methanol, ethanol, isopropyl alco-A), tertiary-butanol, di-isopropyl ether (DIPE),rtiary-butyl ether (ETBE), tertiary-amylmethylAME), and tertiary-amyl ethyl ether (TAEE).l Regional Water Quality Control Board, 1997).tic Richfield Company (ARCO) synthesizedtertiary butyl ether (MTBE) in 1979 via thec reaction of isobutylene ((CH3)2C55CH2) andol (CH3OH) (American Petroleum Institute,MTBE usage increased rapidly in the 1980s at40% per year (Suflita and Mormile, 1993;et al., 1997). In 1992, the actual productionE in the United States was 9.1 billion pounds., MTBE was the most widely used oxygenate inited States although TAME, ETBE and DIPEso blended with gasoline. In addition to thisic production, Alberta Envirofuels in Canadago in Argentina have imported MTBE into theStates.E was initially added as an octane-enhancingment for tetraethyl-lead since the mid-1980s. It

wasbenzdistaW

tificabiasExa

. poteva(DV

. pobe

. ingatrprim

. cres

. diMtance of about 600 feet, however, is consistentsecond release. Corroborative evidence wasently obtained to verify the location of thissuspected release.lenges to this interpretation include the possib-t dierences in alkyl-lead and lead scavengertrations are attributable to dierences in subsur-eathering patterns and product comminglingthan multiple releases. An additional compli-ay include sampling and analytical variations

lkyl-lead and lead scavenger concentrations as an of the gasoline thickness. A discontinuous thine layer, for example, is exposed to greater waterper volume of product than a layer of gasolinefeet thick. A sample collected from a thickere layer may therefore exhibit higher EDB and/ord EDB/EDC mass concentrations as a functionuct thickness. Graphing product thickness v. thelkyl-lead/EDB ratio may therefore provideas to whether this is a reasonable explanation.r releases, the alkyl-leads may be absent as theyngly adsorbed to soil and quickly hydrolyzentact with groundwater. In such cases, only thevengers may be present.

nates

ates are blended with gasoline to increase thecontent and reduce carbon monoxide emis-he American Society for Testing and Materialsan oxygenate as An oxygen-containing,organic compound, such as an alcohol orhich can be used as a fuel or fuel supplement1998). In the United States, ethanol waswith gasoline in the 1930s and 1940s, althoughspread use did not occur until after 1978. Thee of blending ethanol with gasoline was tothe octane quality and as a fuel extender.

and(Daydiscran in2000howwasozonozontratiMTBMTBMidCoa1996wasFedeoxygning1995stanAgeusagMorM

( 4soilfreqwateethybehaof thM

thansandgrou75%

D. Morrisoner used as a fuel oxygenate to decrease thet of carbon monoxide in automobile emissions,

. prodexchaimprove the tolerance of gasoline for moisture000). Currently, MTBE is used as evidence toinate between multiple spills of gasoline and ascator of a pre- and post-1979 release (Morrison,avidson and Creek, 2000). MTBE is not,

r, contained in all post-1980 gasoline. MTBEended with reformulated gasoline for severeon attainment areas that did not meet federalambient air quality standards with concen-of 15% (Oxy-fuels) while many states usedas an octane booster at up to 8% by volume.was introduced to East Coast, Gulf andt gasolines after 19791980 and into Westasoline after about 19861987 (Squillance et al.,avidson and Creek, 2000). Since the 1990s, ited in gasoline in over 15 states to meet theClean Air Act of 1990 requirements for

ates in wintertime oxygenated gasolines (begin-1992), and in Federal reformulated gasolines inmeet carbon monoxide ambient air quality

ds (California Environmental Protection, 1996). Table 1 is a chronology of oxygenateibbs, 1998; Harvey, 1998; Morrison, 1999d,

on, 2000a,c).E is about 25 times more soluble in water00 mg/L) than benzene, and is not retarded byit travels in groundwater. MTBE is thereforetly detected at the leading edge of a ground-lume in the absence of the benzene, toluene,nzene, and xylenes (BTEX). As a result of thisr, MTBE may be used as a qualitative indicatorength of the downgradient plume.E plumes in groundwater are generally longerTEX plumes. In field studies of unconfinedquifers, MTBE migrated at the same rate aswater while BTEX migrated at about 90%,nd 67% of the groundwater velocity, respect-t a site in South Carolina, MTBE in gasolinensported at the same rate as groundwater whilee was transported about 80% of the samee (Landmeyer et al., 1998).n MTBE is used for age dating and source iden-n, it is important to examine whether potentialre present that can impact the interpretation.les of biases include the following:

tial false positives (13%) in laboratoryg when EPAMethod 8020/8021 is used becauseus methyl-pentanes co-elute with MTBEidson, 2000a; Uhler et al., 2000; Rhodes anduyft, no date);tial contributions from non-point sources mustentified (Delzer et al., 1996);ental blending and/or mixing of additives inine suppliers as gasoline without deposit con-additives is exchanged or traded amountucers to meet contract requirements and toove transportation logistic;contamination from one fuel to another,

ially in pipelines and tanker trucks;ences in MTBE concentrations or absence ofE due to seasonal reformulations;

uct swapping by the gasoline jobbers, or viange agreements between refineries or bulk

200

is i

1).plic

lineuels

s, 1atesmenha

ols STable 1. Chronology of oxygenate usage

Date Description of oxygenate usage and history

1842 MTBE synthesized by English chemist (Faulk and Gray,1907 tertiary-Amyl methyl ether produced.1930 Agrol, alkylgas, ethanol fuels used in Nebraska.1933 tertiary-Alkyl ether synthesis patent in the United States1937 Germany uses methanol (Harvey, 1998).1940 Alkyl-Gas (ethanol blend) marketed in Nebraska.1943 Patents relating to MTBE filed (U.S. Patent No. 1,968,601950s American Petroleum Institute literature references the ap1968 Chevron taxicab field test of MTBE/TAME1969 ARCO Corporation blends tertiary butyl alcohol in gaso1973 MTBE used in commercial gasoline in Italy (Hart/IRI F1974 Clean Air Act Amendment requires waivers.1975 Nebraska gasohol (ethanol blend) program commences.1976 EPA waiver issued for 10% by volume for ethanol (Gibb1979 MTBE produced by ARCO Corporation in the United St

volume for MTBE. EPA waiver issued for 2.5% each foreastern seaboard, from 1979 to the mid-1980s for octane e1986; Weaver et al., 1996; Davidson, 2000b).

1980s Experimentation with MTBE, methanol (M85) and ethanon the East Coast of the United States pump MTBE. EPAstorage facilities (Hitzig, Kostecki and Leonard,1998; Rhodes and Verstuyft, no date).

Given the high volatilization potential (245 mm Hg @258C) the solubility of MTBE in water, its presence ingroundwater may not be indicative of a liquid release.The presence of MTBE in groundwater may originatefrom a non point source, especially at concentrations ofless than 10 mg/L (Pankow et al., 1997). Non-pointsources of MTBE include storm water runo(015 mg/L) and surface water sources such as water-craft (040 mg/L) (Davidson, 1999). The United StatesGeological Survey, for example, detected MTBE in6.9% of 592 water samples collected from storm waterin 16 cities and metropolitan areas at concentrationsranging from 0.2 to 8.7 mg/L (Delzer et al., 1996).Eighty-three percent of the MTBE detections occurredin storm water that was collected from Octoberthrough March (19911995) and corresponded to theseasonal use of oxygenated gasoline which happenswhen carbon monoxide exceeds established air-qualitystandards.

limit (11% by volume for MTBE).1982 Documented use on the East Coast of the United States (Ka1987/1988 Denver begins first wintertime oxygenated gasoline program

emissions (ethanol subsequently used) (American Petroleum15% by volume for MTBE as the maximum amount; MTB(Uhler et al., 2000).

1989 Phoenix, Las Vegas, Reno, and Alburquerque begin wintertClean Air Act Amendments enacted. The Substantially Similaby volume for MTBE).

1992 Oxygenates required during the winter in carbon monoxidwintertime oxygenated gasoline program requires 2.7% by w

1993 Tertiary amyl methyl ether and ethyl tertiary butyl ether usozone non-attainment areas. Federal reformulated gasoline prPhase 2 requires reformulated gasoline and requires 1.82California contains MTBE. MTBE is the most widely used o

1995 TAME added to California fuels since 1995 (Central Region1997 Approximately 8 billion kilograms of MTBE is produced in1998 California Health and Environmental Assessment of MTBE

gasoline; 3.8 billion gallons of MTBE used in the United Statepilot project in response to reports of widespread MTBE co

1999 Chevron and TOSCO begin gradual phase-out of MTBE in ubecause of concerns about its potential impact on the town

2000 California bans the use of MTBE with a complete phase-ouCalifornia Department of Health Services establishes an MCeective on 18 May, 2000.0).

ssued.

ability of using MTBE in gasoline (Drogos, 2000).

(Harvey, 1998; Drogos, 2000; Peterson, 2000)., 2000).

998).(Davidson and Creek, 2000; Bruya, 2000). EPAwaiver issued for 7%thanol and tertiary butyl alcohol. MTBE included in gasoline in thencement (McKinnon and Dyksen, 1984; Garrett, Moreau and Lowry,

as octane boosters conducted (Harvey, 1998). Transmission pipelinesubstantially Similar rule issued with 2% by weight oxygen maximum

Review of Environmental Forensic Techniques 179Cross-contamination of non gasoline products withMTBE can occur during shipping and storage inpipelines, tankers, above-ground storage tanks, andtrucks. MTBE has been detected in the presence of jetfuel, diesel fuel, heating oil, aviation gas, and waste oil,presumably due to cross-contamination (Hitzig,Kostecki and Leonard, 1998). In Connecticut, 27 of37 heating oil spill sites had MTBE detected in thegroundwater at concentrations from 1 to 4100 mg/L(Davidson, 1999).Biodegradation studies of MTBE in shallow aquifers

indicate that MTBE is biodegradable under anaerobicand aerobic environments although it is consideredrecalcitrant relative to BTEX compounds (Mormile,Liu and Suflita, 1994; Landmeyer et al., 1998;Ramsden, 2000). A suspected MTBE degradationpathway is shown on Figure 2.While the author has not encountered a case

where MTBE and degradation product ratios areused for age dating and/or source allocation, a greaterunderstanding of the kinetics associated with thesepathways may provide a means for developing this

plan and Galperin, 1996; Kaplan et al. 1997).in the United States using MTBE to reduce vehicle carbon monoxideInstitute, 1998; Harvey, 1998; Drogos, 2000). EPA waiver issued forE is among the top 50 chemical manufactured in the United States

ime oxygenated gasoline program using MTBE (ethanol used later).r Rules increases the maximum oxygen limit to 2.7% by weight (15%

e non-attainment areas. Ethanol used where economical. Federaleight minimum oxygen in 39 carbon monoxide non-attainment areas.age comes into general use (Peterson, 2000). Reformulated gasolineogram requires 2.0% by weight minimum oxygen content. California.2% by weight oxygen. Ninety-five percent of all gasoline sold inxygenate in the United States (Reisch, 1994).al Water Quality Control Board, 1997) .the United States (Hitzig, Kostecki and Leonard, 1998).report recommends the gradual phase-out of MTBE in Californias (Drogos, 2000). TOSCO blends ethanol into California gasoline as antamination on groundwater.nleaded gasoline. Town of South Lake Tahoe, California bans MTBEs drinking water supply.t by 31 December 2002 (Brown and Clark, 1999). 20 March 2000,L for MTBE at 13 mg/L; a secondary MCL standard is set at 5 mg/L,

argument. A potential diculty is that while TBA is aprimarymicrobsynthesMTBEmay al(Browntherefoship wicomm.)use in i

Dyes

The incagreemCorpor1926. Tof dyegasolinpurposwhethegasolintions agradesdyes adierenare usegasolin(alkylorangeaminoanoanthKaplandye ran(Younggasolinantikno

are addhedhe21raqomzelayrptnaysisorm

ol

ylat

metabolism

O

C

OH

Met

Figure 2. Simplified methyl tertiary butyl ether (MTBE) oxidation pathway (after

180 R.metabolite of MTBE via atmospheric andial oxidation, it is also a by-product of MTBEis and is often present as an impurity within the 0.1 to 1% range (Drogos, 2000). TBAso be added to MTBE up to 5% by volume, 2000). The detection of TBA with MTBE mayre be independent of its degradation relation-th MTBE (Wilson et al., 2000; Ian Kaplan, pers.. MTBE has also been proposed as a tracer fornverse groundwater modeling applications.

finisT

foranthIn sbronthinabsoity aanalperfMTBE

TBA

Lactate

2-Propan

Methylacr

Formaldehyde

O

O

C C

C

C

CH

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CH3

CH35

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3 CH3

CH3

CH2OC1

H2O

O2 + 2H+

OH

OH

D. Morrisonlusion of dyes in gasoline was the result of anent concerning tetraethyl-lead between Ethylation and the United States Surgeon General inhe agreement required that a sucient amountbe added to impart staining qualities to leadede to deter its usage for cleaning or otheres. Dyes are used by some states to determiner the highway tax was paid and/or if thee is used for non highway (agricultural) applica-nd blended by refiners to distinguish betweenof fuel. For example, yellow/gold and pink/redre added to gasoline to distinguish betweent grades of gasoline. Blue, yellow, and red dyesd to dierentiate octane ratings in aviatione (Ward, 1984). Common dyes include red

derivative azobenzene-4-azo-2-naphthol),(benzene-azo-2-naphthol), yellow (para-diethylzobenzene), and blue (1,4-di-isopropyl-ami-raquinone) (Kaplan and Galperin, 1996;et al., 1997). Typical concentrations for a dryge from 0.7 to 1.3 grams per 100 gallonslass et al., 1985). Commercial dyes used ine (15 mg/L) are usually part of a leadck package containing lead scavengers that

Whilgasolinpatentmigratialteratiorangefuel haset al., 1distingublendsbrown)subsurfsubsequand the

Stable

Radioaof petrand toespeciaIsotopemany cof lightthan hefor agehydroced at the refinery at the in-line blender or thefuel-blending tank.high-resolution mass spectral chromatogramscommercial dyes suggested azo-, diazo-, anduinone-type structures (Younglass et al., 1985).e cases the dye was multi-component (e.g. adye). Dyes in gasoline are analyzed with eitherer chromatography or by ultraviolet or visibleion spectroscopy (Touchstone, 1992). Special-lytical laboratories that have experience in thisare recommended if this analysis is to beed and relied upon as evidence.

e

abolism

Drogos, 2000; Ramsden, 2000; Jacobs et al., 2000).O

O

Intermediarye dyes have been used for more than 55 years ine, little information exists on dyes other than indisclosures. Furthermore, the subsurface

on of a fuel containing dye can result in theon of the original color. For example, red anddyes can assume a dark brown color after themigrated a short distance through soil (Kaplan997). Other challenges include the diculty inishing between dyes when several gasolinecommingle (red and orange dyes become dark, the rapid biodegradation of the dyes in theace (probably due to hydrogenation andent destruction of the conjugated structure),ir high water solubility (Galperin, 1997).

Isotope Analysis

ctive isotopes are used for source identificationoleum hydrocarbons and chlorinated solventsestimate contaminant transport travel times,lly for single component spills (Philp, 2000).ratios are less aected by weathering than arehemical concentration ratios, and isotope ratioser fractions are more susceptible to weatheringavier fractions. Numerous opportunities existdating and source identification of petroleumarbons with isotope analysis. An advantage in

using isotope analysis for petroleum hydrocarbonfingerprinting is that the isotopic results are similar tothe original hydrocarbon and can be used tocomplement gas chromatography and/or gaschromatography/mass spectrometry findings (Mansuy,Philip and Allen, 1997). The stable carbon isotopecomposition has been used to distinguish betweengases from dierent sources and whether they are ofmicrobial or thermogenic origin (Philp, 1998). Anotherapplication of isotope analyses was reported forC29/C30 ratios and S C31-C35/C30 ratios to distinguishMiddle Eastern v. South East Asian petroleum in theStraits of Malacca, Malaysia (Zakaria et al., 2000).Lead isotope analysis is used for age dating and

source identification of gasoline releases. Lead radio-active isotopes are usually reported as ratios of 206Pb/204Pb, of 206Pb/207Pb, or as a delta notation (d). Isotoperatios are assigned a negative notation if the samplevalue is lower than the standard value (arbitrarily givenas 0%), or they are reported as a positive value if thesample ratio is greater than the standard value. Anexample of delta notation is (Hurst, 1998a,b):

d206Pb 1000206Pb=207Pbsample 206Pb=207Pbstandard=206Pb=207Pbstandard:

1

The most frequent lead isotope ratios used for agedating are 206Pb/207Pb and 206Pb/204Pb (Hurst, Davisand Chinn, 1996). When 206Pb/207Pb ratios for tetra-ethyl-lead are plotted as a function of time between thelate 1960s and the late 1980s, for example, a systematictrend is observed that is alleged to be a result ofmanufacturers shifting their source of lead. A tech-nique relying upon this approach is the Anthropogenic(i.e. gasoline derived) Lead Archeostratigraphy Model(ALAS) that is used for source identification and agedating a gasoline release(s). This approach is based onthe observation that the average stable isotope ratios ofleaded gasoline were relatively uniform over intervalsof one year. From 1964 to 1990, the 206Pb/207Pb ratiosin United States gasoline and aerosols were measuredand observed to reflect a characteristic isotopicsignature (Rosman et al., 1994). Dierences in theisotope ratios of the ores used for gasoline leadpackages provide the basis to compare lead isotoperatios from environmental samples for age dating andsource identification.From 1902 to about 1968, most industrial lead

emissions in the atmosphere originated from geologi-cally old lead ores with 206Pb/207Pb ratios betweenabout 1.141 and 1.167 (Erel and Patterson, 1994).From 1968 to 1978 an abrupt change in these ratiosoccurred because the major lead source in the UnitedStates shifted to younger ores mined in Missouri thatpossessed anomalous high 206Pb/207Pb ratios of about1.35. These ores constituted about 9% of the totalindustrial consumption in 1962, but increased to 27%in 1968, to 57% in 1971, and to 82% by 1976. Since1984, 206Pb/207Pb ratios decreased to about 1.18, and to1.2 in 1989/1990 (Stukas and Barrie, 1987).Evaluation of lead isotope ratios and lead concentra-

tions provides the basis for comparison with the ALAS

model calibration curve. By plotting isotope ratios, or alead isotope ratio v. lead concentration, patterns ofdata can allegedly be identified and used for sourceidentification. It is reported that this technique allowsone to establish the time of formulation to within 1 to 5years. This ability is dependent upon the slope of theALAS model curve, the calibration sample scatter, andthat the sample is an end member and not a mixedisotope signal due to the preferential removal ofanthropogenic lead relative to natural lead (Hurst,2000). The degree of resolution for age dating isreported to be+1 year for releases from 1965 to 1980,and +1.52 years for 1980 to 1990, with the largerdeviations occurring for post-1985 results (Cline,Delfino and Rao, 1991; Hurst, 1998a,b, 1999a,b).After 1990, when gasoline became unleaded in theUnited States, age estimates can only be stipulated aspost-1990. Parts per billion (ppb) levels of leaddetected in unleaded gasoline post-1990 are assumedto be attributable to inherited lead from the crude oiland refining process (Hurst, 1998a). In instances where206Pb/207Pb ratios are indistinguishable, the use of206Pb/204Pb ratios may provide the necessary discrimi-nation.Unleaded gasoline can contain lead concentrations

in ppb range. The concentrations detected are assumedto be representative of the geologic formation fromwhich the lead was obtained that is contained within anadditive package as well as from the refinery where thegasoline was produced (Hurst, Davis and Chinn, 1996).The ALAS approach assumes that the lead additive

producers, Ethyl Corporation and E.I. DuPont deNemours, used similar ore feedstock for their alkyl-lead additive packages, and that the same lead additiveproportions were similar for any given year. Anotherassumption is that the environmental sample is not amixture of multiple gasoline releases with dierent leadadditives (Hurst, Davis and Chinn, 1996). Otherchallenges include whether the standard used in theALAS model discriminates between industrial leadoriginating from unleaded gasoline v. lead from otherindustrial sources. For example, lead concentrations inorganic industrial sewage particles range from 500 to1200 mg/kg (dry weight) that can be introduced intosediments in addition to lead originating from auto-mobile exhaust that are used as a standard for dating agasoline release (Patterson and Settle, 1976). Potentialchallenges and responses are summarized in Table 2(after Hurst, 2000).Carbon and hydrogen isotope analysis is also used

for crude oil and for aromatic compounds (BTEX)source identification. The low 13C values of fuels andchlorinated solvents manufactured from fossil fuels candier with the dissolved inorganic carbon in naturalgroundwater. The d13C of the dissolved organic andinorganic carbon provide a basis of contrast in areasimpacted by chemicals produced from fossil fuels. Ifeach refinery processes crude oil from a particulargeographic area or oil basin for an extended period oftime, the isotope ratios for the corresponding refinedproduct are expected to be similar. If dierent crude oilstocks are blended at the refinery, small changes in thecarbon isotope ratios of the refined fuels can result.The changes in the isotope ratios usually occur duringthe production of the light gases that tend to

Review of Environmental Forensic Techniques 181concentrate heavy isotopes in the product (Kaplanand Galperin, 1996).

Dieprovidesourcesmost cobe com(PNA)of crudwithoutinguishused mmay alwith ening theIsoto

promisiof gasoassumedistinctcharactsition isbe usedbackgroPenta

subsequchromatrometrdiscrimratios oconcensampleIRMSdierentinct ddierenplots oprovidesourcesto spatBTEXWhe

poundsexaminindivid

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ath

oleoration, water washing, adsorption and/or seques-on,ctisferndern1) asoda

Table 2. rapgasoline r

Challeng nse

Lead concertain tconcentra

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Single, dleaches a

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leadgaounvia

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182 R.rences in sulfur isotopes (32S, 33S, 34S, and 36S)an opportunity to distinguish between dierentof crude oil or product. 32S and 34S are themmonly used isotopes for this purpose and canbined with a peak-to-peak polynuclear aromaticanalysis to distinguish between multiple sourcese or refined products. PNA analysis, with ort stable isotope analysis, can assist in dis-ing between pristine and used motor oil asotor oil contains more PNAs. Used motor oilso be diluted with fuel (1 to 10%), especiallygines with worn piston rings thereby complicat-interpretation of this data (Rhodes, 2000).pic analysis of the BTEX compounds presents ang opportunity for identifying discrete releasesline from multiple sources. These approachesthat hydrocarbons enter the subsurface with aisotopic composition, or 13C/12C ratio, that iseristic of their source. If this isotopic compo-conserved, stable carbon isotopic analysis can

for abe cComBTEquansour(Dem

We

Petrevaptratiadvetrangroupattto: (monage

Challenges and responses to use of the Anthropogenic Lead Archeostratigeleases

e to ALAS model Respo

centrations in soil and groundwater that do not exceed ahreshold concentration are not attributable to backgroundtions and are indistinguishable from anthropogenic lead.

High pnatura

ilute acids used in sample preparation for soils preferentiallynthropogenic lead.

Soil sathe resthe lea

topes fractionate or biodegrade, thereby compromising theirn for age dating and source identification.

Masspreclud

ling of hydrocarbon releases homogenizes lead isotope Whensolublethe grderived

bsent in unleaded gasoline. Leadhundreproduc

D. Morrisonto identify dierent sources as well as naturalund sources of hydrocarbons.ne extraction of a groundwater sample andent analysis of the d13C via high sensitivity gastograph/combustion/isotope ratio mass spec-y (GC/C/IRMS) may be appropriate for sourceination of BTEX in groundwater as isotopef d13C remain constant as a function of thetration of the BTEX in dissolved or free products (Dempster, Lollar and Feenstra, 1997). GC/C/analysis of pure phase BTEX indicates thatt manufacturers produce chemicals with dis-13C compositions probably attributable tot feedstocks and refining processes. Cross-f deuterium and hydrogen ratios may similarlya means to distinguish between multiple BTEX. A similar technique for source identification isially examine enrichment of d13C values in thecompounds in a commingled plume.n reviewing d13C values for the BTEX com-for source identification, it is important to

e the overall pattern of isotopic variation (e.g.ual BTEX plots). A characteristic d13C signature

sideredcarbonmicrobrelative2000).The

pentaneto deterwith gamay inarguedsamplen-heptahexaneMCH cC7 normfor comoften dage daspecificwashingBiom

presentchemical precipitation, biodegradation, andve transport. Water washing is the preferentialof the BTEX and light hydrocarbons into

water. The merit in examining weatherings among samples is that it may provide a basisssociate products/fuels originating from a com-urce; and (2) provide a qualitative basis forting. Toluene, for example, is generally con-mpound such as benzene, for example, may notrly associated with a particular manufacturer.und specific isotope analysis for all of thecompounds may therefore provide a moreable basis to distinguish between multipleas contrasted with using a single compound

ster, Lollar and Feenstra, 1997).

ering and Biomarkers

um hydrocarbon weathering processes include

hy Model (ALAS) for age dating and source identification of

ision lead isotopic ratio analyses can discriminate betweend anthropogenic lead at the lower ppb concentrations.

es can be subjected to multiple sequential extractions withinterpreted via rigorous mixing models to accurately identifyotopic-concentration characteristic of each end member.

erences in lead isotopes are small (0.52%) therebyfractionation via physiochemical and biological processes.

concentrations are greater than about 10 ppb and water-soline compounds are present, the lead isotopic signature indwater is virtually identical to the anthropogenic leadhydrocarbon/groundwater lead isotopic exchange.

be present in unleaded gasoline at levels from tens off ppb. The origin of this lead is the crude oil from which thesoline was produced and from the refining process.to be more biodegradable than other hydro-s in gasoline and can therefore indicateial degradation when preferentially removedto benzene and the C8 aromatics (Peterson,

presence of the gases isobutane, n-butane, iso-, and n-pentane can provide a qualitative basismine the relative age of a sample contaminatedsoline. The presence of these gases in a sampledicate a fresher sample while their absence isas indicative of an older or more weathered. Fresh gasoline also contains n-hexane andne in higher concentrations than methylcyclo-(MCH) and n-octane. As gasoline weathers, theoncentration increases relative to n-hexane, andal parans, thereby providing a relative meansparison between samples. Forensic geochemistsisagree as to the validity of using weathering forting gasoline, especially given the many sitevariables such as volatilization and watereects (Peterson, 2000).arkers are defined as organic compoundsin oils and source rocks having carbon

taneous and 100% eective regardless of location.skeletons related to their functionalized precursorswhich occur in the original source material (Philp,1998). Most crude oil and Bunker C fuel, for example,contain biomarkers such as terpanes and steranes thatare highly resistant to biodegradation (Walker, Colwelland Petrakis, 1976; Kaplan et al., 1995). The moleculardistributions of terpanes and steranes and carbonisotope values, for example, was used to dierentiateExxon Valdez-derived oil from California oils thatwere introduced into the Alaskan region prior to the oilspill. In another application, hopane and steranedistributions, in addition to microscopic characteriza-tion, were used to identify road asphalt particles as asource of contamination in river sediments fromAlsace-Lorraine (Faure et al., 2000). Individual bio-markers and ratios used for source identification ofhydrocarbons include C2-debenzothiophenes/C2-phenanthrenes, C3-dibenzothiophene/C3-chrysene, andC29 a, b-pentacyclic hopanes/C30 , a, b-pentacyclichopanes, C23 tricyclic hopanes/C24 tricyclic hopanes,and 4-methyldibenzothiophene/2- and 3-methyldiben-zothiphenes ratios (Wang, Fingas and Sergy, 1994;Wang and Fingas, 1995; Douglas et al., 1996). The C30pentacyclic terpane (hopane) and certain tricyclicterpanes are among the most stable biomarkers incrude oil (Peters and Maldowon, 1993). Of thetetracyclic steranes, the diasteranes are the most stable.In environmental litigation, pentacyclic triterpanes(C27C35) and steranes (C27C30) are commonly used,although they are relegated to use for distinguishingbetween crude oil or heavy distillate fuels due to theirhigh molecular weight (Stout et al., 1999a). For middledistillate fuels, biomarkers using gas chromatography/mass spectrometry-selected ion monitoring techniquesare used to distinguish dierent fuels. Biomarkersidentified with this instrument include bicyclic sesqui-terpanes (C14C16), acyclic regular isoprenoids(C13C25), tricyclic diterpanes (C17C20), aromaticditerpenoids (C18C20), tricyclic terpanes (C19C25),and various polycyclic aromatic hydrocarbons such asnaphthalenes and phenanthrenes (Stout, Uhler andMcCarthy, 1999; Stout et al., 1999a). Dibenzothio-phenes are associated with the sulfur content of the fueland can vary significantly between sources.Crude oil and most mid-range distillates contain an

abundant number of PNAs including naphthalene,fluorine, phenanthrene, pyrene, and chrysene (Kaplan,2000a). Manufactured gas plant residues containingPNAs include lamp black, tar and/or spent oxides(usually composed of sulfur, cyanide and ammoniacompounds bound with iron). The 1984 EdisonElectric Companys Handbook of Manufactured GasPlant Sites describe the composition of a coke plantscoal tar consisting of 5% light oil (boiling points up to2008C), 17% middle oil (2002508C), 7% heavy oil(2503008C), 9% anthracene oil (3003508C), and62% pitch (43508C). The presence or absence of aPNA may therefore provide an indication of adistillation or pyrogenic process that is unique to thematerial and can be used to distinguish between residuematerials containing the PAHs. Care is required whenperforming this analysis, as the chemical compositionof the coal and tars used as feedstock vary significantly,

including dyestu feedstock chemicals (e.g. phenols,creosols, xylenes, parent and alkylated PNA), oil cutsAn example of a method used to estimate thedegradation (l) of a compound in a one-dimensionalidealization is described by (Buscheck and Alcantar,1995; Brown et al., 1997; Westervelt et al., 1997):

l vc=4ax1 2axk=vx2 1 2

where vc is the contaminant velocity along thex-direction (adjusted for retardation), ax is thelongitudinal dispersivity coecient, k is the attenua-tion rate in units of time, and vx is the lineargroundwater velocity. The term (k/vx) describes theslope of the regression line fit to the log contaminantconcentration data as a function of distance along thecenterline of the contaminant plume (McNab andDooher, 1998). The diculty in relying on this inversesolution relationship is that dispersive processes canproduce concentration distributions that decline withdistance from a continuous source. In many instances,(e.g. light oil, naptha, heavy naptha, naphthalene oil,wash oil), and residual materials (e.g. light creosote,creosote, heavy creosote and pitches). A comparison ofthe PNAs present in a sample may provide insight todistinguish between sources that used dierent feed-stocks (e.g. crude oil, coal-tar, oil tar, etc.) (Haeseleret al., 1999).When using biomarkers for age dating and/or source

identification, it is important to examine the back-ground concentrations of those compounds selected asbiomarkers (Stout, Uhler and McCarthy, 2000). Therefining process may also aect the use of biomarkersto some degree. For straight-run distillate products,biomarkers are impacted only by the distillationtemperature, while in cracked and hydrotreated pro-ducts, biomarkers are aected by temperature, cata-lysts, and hydrogen. The potential impact, if any, of therefining process on a selected biomarker thereforerequires examination.

Degradation Models

Degradation models used for age dating are basedupon a data set for which a particular degradation rateis postulated. This degradation rate is then used topredict the known concentration of compound for anearlier period of time (Ram et al., 1999).A popular degradation model is based on the half-life

of a compound. The basis of this approach is relianceon the biodegradation half-life. For example, numerousdegradation rate models exist for age dating BTEXcompounds. Assumptions used in first-order BTEXbiodegradation models include (Odermatt, 1994):

. a uniform degradation rate;

. a first-order degradation rate independent of the in-situ microbial population;

. omission of the contaminant loading rate and thetoxic eects of the contaminant on the microbialpopulation (e.g. first-order degradation rates mayonly be valid over a portion of a concentrationrange);

. a first-order biodegradation process that is instan-

Review of Environmental Forensic Techniques 183especially when analyzing small numbers of datapoints, it is often possible to fit a straight line through

log concorrelatabsent.functiohowevetransfoaccounfunctioincludeexist, flFickianflow anwith thdue to wand a(McNaThe

hydrocaappropgroundapplicaapproprecommapproxis greatGodsy,

Prista

PristanisoprenbetweenThey archromalubricatlubricat 17 18more prominent than the pristane and phytane peaks.As thebacteriaresultsmore pof the Cas a qudegrada(Kaplaextendi

where adispens(KaplaThe

the agNetherauthorscompanmethodpeak hdieren2000b).ratios wanalysisage datdistillat

indarede oheineablmininen-

theiese se Un

plerelsidpilodlenodmmor

X

XtifiXl-leartiqutilizXen

gasolineneence occurs with BTEX in soils. Toluene, ethyl-enivee rehein

lyalthth. TT=nentg

TEaher.prein tatiGa

184 R.petroleum hydrocarbon is biodegraded, thepreferentially consume the C17 and C18 that

in the pristane and phytane peaks becomingronounced on the chromatogram. Examination

17/pristane and C18/phytane is therefore arguedalitative basis for determining the degree oftion and hence weathering, primarily for dieseln and Galperin, 1996). A linear relationshipng for about 20 years is described by:

Tyear 8:4n-C17=Pr 19:8

n average initial value for [n-C17/Pr] ratio ined diesel fuel is about 2.3 for No. 2 diesel fueln et al., 1995, 1997).pristane/phytane method was used to estimatee of a No. 2 diesel in Denmark and thelands (Christensen and Larsen, 1993). Theanalyzed 11 diesel fuels from five dierent oilies using dierent laboratories and analyticals. The peak heights were used for ratios (e.g.,eights from a baseline can be dierent fort GC/FIC systems and methods) (Smith,The authors concluded that the average C17as 1.98+2.0. The authors concluded that anof the C17/pristane ratios provided a means to

tolusequbenzrelatmorT

andrareinitithatfromratioB expo(Mo

Bafteron twateinterwithindicandcentration v. distance data with a high degree ofion even when degradation is insignificant orA linear trend in log concentration values as an of distance from the contaminant source,r, does not constitute proof of the existence ofrmation processes. Other factors that cant for the linear trend in log concentration as an of distance from the contaminant sourcethe assumption that steady-state conditionsuctuations in source strength with time, non-dispersion of solutes, strongly heterogeneousd transport, well locations that are not alignede contaminant plume centerline, dilution eectsell screen length, sampling and analytical bias,nonuniform degradation rate distribution

b and Dooher, 1998).use of first-order reaction rates to describerbon biodegradation may not be universallyriate. Examination of 1029 leaking under-storage tank sites in California found that thebility of first-order degradation rates wasriate in only about 625 instances. The authorsended carefully examining first orderimations if the maximum benzene concentrationer than or equal to 1 ppm (Bekins, Warren and1998; McNab and Dooher, 1999).

ne/Phytane Ratios

e and phytane are isoparans known asoids whose ratio has been used to dierentiatesources of crude oil (Zakaria et al., 2000).

e present to the right of C17 and C18 peaks ontograms of crude oils, middle distillates, anding oils. In fresh crude, middle distillates oring oils the n-paran C and C peaks are

examstanrefincrudW

pristvaluassupristthanwhefor da sitin thknowsamtheConthe smethchalmethrecoand/

BTE

BTEidenBTEalkya parevolaBTEbenz

D. Morrisone the diesel. The ratios for 26 refined and freshes and motor oils from the United States were

ethylbehighere, and xylenes are preferentially retained by soilto benzene. Ethylbenzene and xylenes are alsosistant to degradation than benzene or toluene.initial BTEX concentrations in gasoline varyitial BTEX ratio values in the product areavailable. A technique that addresses theseconcentration variations and the challengesese processes preferentially remove benzenee groundwater is to use a cumulative BTEXhe cumulative BTEX ratio is defined as Rb E X and assumes that the ratio decreasesntially with time after a release, according toomery, 1991; Kaplan et al., 1995, 1997):

Rb 6:0exp0:308 t:

X partitioning studies indicate that immediatelyspill, Rb values range from 1.5 to 6, dependingamount of gasoline in contact with ground-A value between 1.5 and 6.0 is generallyted as indicative of a release that occurredhe last 1 to 5 years, while a value less than 0.5 isve of a release of greater than 10 years (Kaplanlperin, 1996). The ratio of benzene plus toluene/ed; these C17/pristane ratios were 1.95 with ad deviation of +0.29. A C17/pristane ratio of aproduct such as diesel is greater than 2.0, whileil values range from about 2.0 to 2.1.n evaluating the ratio of normal heptadecane tofor age dating diesel No. 2 fuel oil, it is

e to examine whether there is a scientific basis ing that there are constant degradation rates forand n-C17 , and that pristine degrades slowerC17 (Smith, 2000a). Another area of inquiry isr the Christensen and Larsen C17/pristane ratiosel in Denmark and the Netherlands is valid forpecific distillate, lubricating oil or diesel releasenited States (Schmidt, 1998). In one instance, arelease of hydrocarbons to groundwater wasd from 1993 to 1996 and compared to the age ofease using the n-C17 ratio (Smith, 2000a).erable dierences between the known age ofl and the estimated age occurred when using this, especially as the fuel weathered. Significantges exist regarding the scientific validity of thisand a careful review of these issues is

ended when considering its use for age datingsource identification (Brassell et al., 1981).

Ratios

ratios are used for age dating and sourcecation (Odermatt, 1994; Kaplan et al., 1997).ratio data is usually combined with MTBE,ad or pristane/phytane analysis as evidence forcular interpretation. BTEX ratio techniquesalitatively based on the sequence of BTEXation and biodegradation. The sequence ofloss in groundwater generally begins withe because it diuses rapidly from phase-separatee and partitions into groundwater, followed by, ethylbenzene and xylenes. The reversenzene xylenes decreases with time due to thesolubility in water of the benzene and toluene

the sample is measured. Absent these direct measure-relative to ethylbenzene and xylenes. The accuracy ofthis technique is improved by using a best-fit regressionline from historical site data.Challenges to BTEX ratio analyses include varia-

tions in the rate of BTEX transformation due tovariations in soil texture, soil mineralogy, microbialdiversity, and electron acceptor availability. Thevolume of the release, groundwater chemistry andhydrodynamic characteristics are additional variables(Alvarez, Heathcote and Powers, 1998; Landmeyeret al., 1998; Peterson, 2000). Other issues include theuncertainty regarding the initial gasoline compositionand chromatographic separation of the BTEX com-ponents during migration through the subsurface. Theinitial BTEX concentration in gasoline is significantbecause the composition of gasoline varies with theoctane rating and with the time of year when thegasoline was formulated. In colder climates, the gaso-line composition changes so that the Reid VaporPressure is high in wintertime to provide easy startup,and low in summertime to prevent vapor lock. Thegasoline grade also aects the initial BTEX compo-sition. Premium gasoline with an antiknock additivepackage generally has a higher fraction of benzenesince it has a higher octane rating. Regulatory changesalso impact the historical composition in gasoline. TheClean Air Act Amendments of 1990, for example,restricted benzene concentrations in gasoline to 1.6%by volume. Gasoline blended prior to 1990, forexample, generally contains more benzene (6% byvolume) than post-1990 gasoline (Johnson,Kemblowski and Colthart, 1990).Separation (i.e. individual BTEX compounds are

transported at dierent velocities through the soil and/or groundwater) can aect BTEX concentrations andhence ratios. The assumption that dierences in BTEXconcentrations are due exclusively to biodegradationwhen separation is occurring may be invalid. Thelocation of a sample relative to the source can impactBTEX values as samples farther from the release aremore susceptible to separation than those closer to therelease.BTEX ratios in dissolved groundwater samples are

aected by the volume and changes in water in contactwith the phase-separate gasoline. Greater volumes ofwater available to water wash the BTEX from thegasoline results in accelerated changes in the BTEXcomposition. The preferential leaching of benzene andtoluene from gasoline decreases the B/X and(B T=E X) ratios in phase-separate productrelative to its presence in groundwater. Reported B/Xratios of 0.2 to 0.9 for water equilibrated withweathered gasoline, for example, are lower than themajority of B/X ratios (Hinchee and Reisinger, 1987).Another challenge to the use of BTEX ratios for age

dating and source identification is that the biodegrada-tion of gasoline, and especially the BTEX components,is highly variable with abrupt changes occurring onmicro and macro scales. Under anaerobic conditions,for example, toluene can degrade faster than benzene.These uncertainties result in a wide range of ratios foridentically aged spills, especially in dierent soils.Another issue to consider when the BTEX compounds

are within a mixture of contaminants is that some ofthe non-BTEX compounds may impact BTEXments, contaminant transport equations are frequentlyused for estimating the transit time. In order to selectthe most appropriate equation(s) that mirror therelease event(s), the most likely transport mechanism(e.g. liquid advection, gas diusion, liquid diusion,and/or evaporation) must be identified.Liquid transport through a monolithic pavement is

assumed to be rapid. This assumption is true if thepavement is cracked allowing unrestricted flow or if therelease occurs over an expansion/control or isolationjoint filled with permeable wood, oakum or tar.Expansion joints are normally located at the junctionof the floor and walls, or foundation columns andfootings. Isolation joints separate a concrete slab fromother parts of a structure to permit horizontal andvertical movement of the concrete slab. Isolation jointsextend the full depth of the slab and include pre-molded joint fillers. Testing the joint material forcontaminants of interest can often establish if acontaminant migrated through the joint material.Paved surfaces

A frequent issue in environmental litigation is whethera solvent migrated through a paved surface such asasphalt, concrete, crushed rock, or compacted soil, andif so, the time required for its transport. Ideally, arepresentative pavement core sample is collected, arepresentative liquid sample is ponded on the pavementin a manner consistent with the circumstances of therelease, and the time required for the liquid to transitdegradation rates (Johnston, Borden and Barlaz,1996; Berg et al., 1999). Variations in the organicmatter in the aquifer can also result in variations in thedegradation rates of BTEX compounds. In aquifers,BTEX biodegradations rates are normally assumed tooccur via first order kinetics where the rate (dC/dt) isproportional to the contaminant concentrationdescribed as lC dC/dt. Given dierences in thesubsurface environment, this assumption may beinvalid.BTEX degradation rates vary according to whether

they are measured in situ or in the laboratory (Chapelleet al., 1996). Laboratory measured biodegradation rateestimates are sensitive to ambient redox conditions andmust be matched to field conditions to obtain reliableresults. The assumption that BTEX componentsdegrade via first-order kinetics may also be inappro-priate when extrapolating laboratory-derived to field-scale degradation rates (Bekins, Warren and Godsy,1998).

Contaminant Transport Models

Contaminant transport models are used for age dating,for source identification, and for cost allocationpurposes. While models are available for contaminanttransport in the air, soil and groundwater, thefollowing text deals only with soil and groundwater.The transport of a contaminant through the subsurfaceis presented according to whether it migrates through apaved surface, soil, or groundwater.

Review of Environmental Forensic Techniques 185Absent direct measurements or the presence ofpreferential pathways, a mathematical model requiring

numeroliquid ptions refluid arinclude

. the ttrans

. a rehydrface;

. physvisco

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re konmeies,omumranl.,lifispoexiquorratecalpotlylatthatvemer tod transport may be dominated by unsaturated flowlting in contaminant velocities several times slowerfotions such as Richards equation may be moreopriate (Richards, 1931). A one-dimensionaless

C@c=@t @=@zK@c=@z @K=@z 9

rer ce m

186 R.us input variables is used to estimate when aenetrated a paved surface. Modeling assump-presentative of the release circumstances ande required for this modeling approach and oftenthe following input parameters:

emporal nature of the release (steady state orient);presentative saturated and/or unsaturatedaulic conductivity value(s) for the paved sur-

ical properties of the contaminant (density,sity, vapor pressure);ical properties of the liquid (phase separate,lved or mixed);mperature at the time of the release;olume of the release;vaporative flux of the liquid into the ambient

erstanding of the circumstances of the releaseement composition is important for developingtic conceptual model. For example, if the modela value for evaporative loss and assumes at liquid thickness on the pavement, the timeed for liquid transport is shorter than iftion is included. Similarly, if cleanup activitiesormed coincident with the release (e.g. sawdust,and, absorbent socks, Sorball, crushed clay,if the spill occurred in a building with forcedse competing activities need to be incorporatede model as they result in a slower calculatedrt rate through the pavement.bles describing the physical condition of theurface include:

her the surface is treated with an epoxy coatingevent corrosion from acid releases (common inl plating shops);her additives such as Dow Latex No. 560 wasd with the concrete to reduce its permeability toicals;ent thickness, porosity, composition and

;ature of the surface prior to the release (i.e. waspregnated with oils and dirt, smooth v. pitted,d toward a drain, etc.).

a pavement sample is available for directlying its porosity and hydraulic conductivity withimilar to the liquid released onto the surface. Ifeasurements are unavailable, estimated valuese properties are available in the literature. Oncentative values for these properties and thetances of the release are defined, a realistictual model can be constructed.contact time of the liquid with the pavement isnt. If phase-separate PCE accumulates in ancrete sump, neutralization pit or clarifier, fore, the residence time and hydraulic gradientsucient for penetration through the concrete.liquid is thinly distributed on the pavement,r, evaporative loss may be sucient to eliminatesibility of liquid migration through the pave-

withat th

@cA

cA

cA

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D. MorrisonOne example of how the evaporative loss ised is the case of an assumed semi-infinite region

matricconducC is the specific water capacity or change inontent in a unit volume of soil per unit changeoisture content, w equals the suction head (i.e.ion for unsaturated flow is:r saturated flow and the use of unsaturated zoneuniform initial concentration and mass transferurface given by (Choy and Reible, 1999):

DAeff=Rf @2cA=@z2 z 2 0;1 3

eff@cA=@zjz0 kacAz; tjz0 t4 0 4

jz!1 cA0 t4 0 5

tjt0 cA0 z 2 0;1 6

ncentration profile is given by:

cA0ferfRfz= 4p

DAeffRft expkaz=DAeff k2at=DAeffRferfc Rfz= 4

pDAeffRft ka t

p=DAeffRfg

z 2 0;1; t4 0: 7

face flux out of the system is described by:

tjz0 kacA0expk2at=DAeffRferfc rka t

p=DAeffRf t4 0 8

a is the surface mass transfer coecient, cA0 iscentration of species A in the air phase per unit, DA(e) is the eective diusion coecient ofRf is the retardation factor, jA is the mass flux ofponent, z is the depth of position, and t is time.erous models are available to calculate the ratesport of a liquid through pavement (Ghadiri1992; Selima, 1998). For saturated flow, aed one-dimensional expression for the verticalrt of a liquid using Darcys Law is available.pression defines the downward velocity (v) ofid as equal to the downward flux (q) divided byosity of the pavement. The downward flux is thed hydraulic conductivity multiplied by thegradient. Saturated hydraulic conductivity

rosity values for paved materials are measuredor can be obtained from published values. Thision results in a value in units of length overat is divided into the pavement thickness toe the transport time.ment transport models using Darcys Lawthat the pavement is saturated with liquidthe release. If the pavement is unsaturated,potential), and K is the unsaturated hydraulictivity value for the soil.

An additional consideration for either saturated orunsaturated flow through a paved surface for a phase-separate hydrophobic fluid, such as TCE, is that theambient hydrophobic pore water in the pavementrepels TCE. While the extent of repulsion is dicult toquantify, the net result is some degree of TCEretardation.An estimate of whether a contaminant vapor cloud

has migrated through a paved surface is a common areaof inquiry, especially when equipment such as vapordegreasers are present. Vapor degreasers are ofteninstalled in a concrete catch basin to capture any liquidspills. While catch basins are eective at mitigatingliquid spills, they exacerbate the potential for vaportransport through the concrete because they act as anaccumulator for the vapor and minimize the potentialfor vapor dilution with the ambient air. Given this spillscenario, the selection of a model assuming that vaportransport is the primary transport mechanism may bemore representative than those that assume liquidtransport. In many cases it is dicult to reconstructthe most dominant contaminant transport phase.Vapor transport models used to estimate the time

required for a chemical to migrate through a pavedsurface usually include the following input variables;vapor density, vapor source flux (constant or transient)above the pavement, the Henrys Law constant for thecontaminant, pavement thickness, porosity and moist-ure content at the time of the release, and theconcentration of the vapor above, within and belowthe pavement prior to the release (Choy and Reible,1999). Numerous equations are available to estimatethe travel time of vapor through pavement (Crank,1985;McCoy andRolston, 1992). Equation 10 describesone approach for the unsteady, diusive radial flow ofvapor from a source (Cohen, Mercer and Matthews,1993):

@2Ca=@r2 1=r@Ca=@r RaD*@Ca=@t 10

where the air filled porosity (na) is assumed to beconstant (see equation (12)), Ra is the soil vaporretardation coecient, Ca is the computed concen-tration of the vapor in air, r is the source radius and theeective diusion coecient, D*, ( for TCE 32 106 m2=sec and 0.072 cm2/sec for PCE) (Lyman,Reehl and Rosenblatt, 1982) is equal to:

D* Dta 11

where ta n2333a =n2t and n2t is the total soil porositywhich is the sum of the air filled porosity and thevolumetric water content. The soil vapor retardationfactor (Ra) is equal to:

Ra 1 nw=naKH rbKd=naKH 12

where nw is the bulk water content, na is the air-filledsoil porosity, rb is the soil bulk density, Kd is thedistribution coecient, and KH is the dimensionlessHenrys Law constant.Another approach is to conceptually consider the

diusion of a vapor through a medium bounded by

two parallel plates. This approach assumes a pavementthickness (l) and a diusion coecient (D) whosesurfaces x 0 and x 1 are maintained at a constantconcentration specified as C1 and C2 , respectively.After a time, steady state conditions are reached and

the contaminant concentration is assumed to beconstant at all locations in the pavement. The diusionequation in one dimension reduces to the followingequation (Crank, 1985):

d2C=d2x 0 13

if the diusion coecient (D) is assumed to beconstant. On integrating with respect of x, thefollowing expression arises:

dC=dx constant 14

and by introducing the conditions at x 0, x 1, andintegrating, then:

C C1=C2 C1 x=l: 15

The concentration changes linearly from C1 to C2 asthe liquid migrates through the pavement. The transferrate of the diusing substance is the same through thepavement described by:

F DdC=dx DC1 C1=l: 16

If the pavement thickness (l) and the surfaceconcentrations C1 and C2 are known, D is deducedfrom an observed value of F. If the surface x 0 ismaintained at a constant concentration C1 and atx 1, evaporation into the atmosphere is assumed forwhich the equilibrium concentration immediatelywithin the paved surface is C2 , so that:

@C=@x hC C2 0; x l 17

then

C C1=C2 C1 hx=1 hl 18

and

F Dh*C1 C2=1 hl 19

and

@C=@x h1C1 C 0; x 0

and

@C=@x h2C C2 0; x l 20

where

C h1C1f1 h2l xg h2C21 h1x=h1 h2 h1h2l:

21

Numerous challenges to liquid and vapor transportof contaminants through a paved surface are available.While these challenges are highly site and modelspecific, generic categories include the following:

. variability of the model parameters;

Review of Environmental Forensic Techniques 187. accuracy of the known circumstances of the spillevent(s);

. envir(e.g.ture,

. consicontaavail

. impaas exmode

Soil

Hundrechemicaequatiopoundzone is1990):

R1@C

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rate of contaminant transport in groundwater isineceosedugsuk,; Wdb; Ntioistede tspohespotraik,tioimouldotes; TmesferNe; C

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term backward extrapolation model wasrteinic

Court,). Stermris19lemicaerttoco

188 R.ity value associated with the diusion of and in water and air is described by theton expression as:

u10=3m =f2m and tG fm um10=3=f2m: 27

resent, the impact of artificial or naturalrt pathways on a model requires scrutiny asn represent a significant source of uncertainty in

1992theMorson,probapplpropolateof aonmental conditions at the time of the release(s)thickness of the spill, composition, air tempera-etc.);stency of the modeled results with measuredminant results under the paved surface (ifable);ct of potential short circuiting pathways, suchpansion joints and/or cracks on the conceptuall.

ds of models describing the transport of al through soil are available. A one-dimensionaln describing the transport of a single com-via advection and diusion in the unsaturated(Jury, Sposito and White, 1986; Jury and Roth,

1=@t Du@2C1=@z2 V@C1=@z lmR1C1 22

1 is the pore water concentration in the vadose

m is the decay constant, R1 is the liquidtion coecient, Du is the eective coecient,is the infiltration rate.retardation coecient (R1) is estimated by:

R1 rbuKdu um fm umKH 23

rbu is the soil bulk density, Kdu is thetion coecient for the contaminant of interest,e soil porosity, um is the soil moisture content,is the Henrys constant for the contaminant of. The distribution coecient (Kdu) of theinant is:

Kdu 0:6foc;uKow 24

oc,u is the fraction of organic carbon in the soil

ow is the octanol-partition coecient of theinant of interest. The degradation rate constantated by:

lm ln2=T1=2m 25

1/2m is the degradation half-life of the contami-interest. The eective diusion coecient:

Du tL DLMKHtGDGM 26

DLM is the molecular diusion coecient intL is the soil tortuosity to water diusionnt, DGM is the molecular diusion coecient in

boricistepermT

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D. Morrisondel results. Examples of artificial and naturalrt mechanisms include dry wells, foundation

tisticalRemsond in 1991 by Allen Kezsbom and Alan Gold-an article describing the Sterling v. Velsicol

al Corporation case (855 F.2d 1188 6th Circuit1988) (Kezsbom and Goldman, 1991; Kornfeld,ubsequent authors describe this approach withreverse or inverse modeling (Erickson and

on, 1994; Woodbury and Ulrych, 1996; Morri-99c; Parker, 2000) and backward random walk (Chandrasekhar, 1943). In its simplesttion, inverse modeling relies upon measuredies or contaminant concentrations to extrap-some early point in time the age and locationntaminant release, most frequently by geosta-ly used as evidence in insurance litigation foridentification, age dating and for cost allocationes. This type of modeling has been primarilyto the realm of environmental consultants,h some peer-reviewed literature is available onbject (Gorelick, Evans and Remson, 1983;1989; Bagtzoglou, Doughery and Thompson,agner, 1992; Skaggs and Kabala, 1994, 1995;ury and Ulrych, 1996; Snodgrass and Kitanidis,eupauer and Wilson, 1999). Models used inn to date contaminant releases range inication from a straight line drawn between ansource and the leading edge of a contaminanto sophisticated three-dimensional contaminantrt models.origin of inverse modeling for contaminantrt in groundwater is predated by research in thensfer literature (Bayo et al., 1992; Huang and1992; Silva-Neto and Ozisik, 1993). Governingns in the heat transfer literature, for example,ilar to the advection-dispersion equations usedndwater models, except that the advection termm used in inverse heat conduction problemsand Lions, 1969; Alifanov and Artyukhin,ikhonov and Arsenin, 1977). Issues regardingter confidence are also addressed in many heatequations (Beck and Arnold, 1977; Alifanovnarokomov, 1989; Marquardt and Auracher,arasso, 1992).

models, utility trenches, sewer or storm water backfill,and septic lines, discrete layers of high

bility soils, and mechanical disturbance.confidence level of model parameters can beed via a probabilistic analysis such as theod (e.g. used for nonlinear models) and Montemethod. Monte Carlo simulations provide acal method for generating simulated data usingnumbers and assumed probability distri-

. This approach also allows the identificationmeters that influence the calculated groundwaterinant concentration, and provide an uncertaintyfor the most sensitive input variables.

waterand optimization approaches (Gorelick and, 1982; Khachikian and Harmon, 2000).

Examples include inverse models for reconstructingcomplex, multi-parameter release histories with knownsource locations from spatially distributed plumeconcentration data (Skaggs and Kabala, 1994, 1995,1998; Woodbury and Ulrych, 1996).One of the earliest applications of inverse modeling

for contaminants in groundwater used a least squaresregression and linear programming to identify thelocation and discharge rates of contaminants thatwere simultaneously migrating from five disposal sitesover a 5-year period (Gorelick, Evans and Remson,1983). Other approaches include identifying the con-taminant source location with backward probabilitymodels to establish the most likely of several releasesites. Bagtzoglou, Doughery and Thompson (1992), forexample, described the use of a reverse time randomparticle method to estimate the probability that aparticular site was the source of a contaminant plume.These probability approaches assume a single pointsource release (Birchwood, 1999). In another appli-cation, the vertical concentrations of PCE and TCE inan aquitard were evaluated to reconstruct a releasehistory; only diusion and sorption were assumed toaect mass transport in the aquitard (Ball et al., 1997;Liu and Ball, 1999). Dimov, Jaekel and Vereecken(1996) used the adjoint formulation of the one-dimensional advection-dispersion-deposition equationfor source identification and loading rate. Anotherapproach modeled contaminants from a constantsource concentration using two-dimensional diusionto estimate source characteristics and transportparameters from head and concentration data(Sonnenberg, Engesgaard and Rosjberg, 1996).Sidauruk, Cheng and Ouazar (1997) developed a setof analytical procedures for contaminant source identi-fication that relied upon contaminant concentrationsmeasured within a two-dimensional contaminantplume. Contaminant flow was assumed to be homo-geneous and groundwater velocity was uniform forinstantaneous and continuous releases. The authorsnoted the limitations of certain parameters that canonly be uniquely determined by sampling at a minimumof two dierent times. Skaggs and Kabala (1994) usedTikhonov regularization (Tikhonov and Arsenin, 1997)to transform the inverse problem into a minimizationproblem with a unique solution as well as using themethod of quasi-reversibility to solve the problem(Skaggs and Kabala, 1995). It is assumed that thelocation of the source is known. Wagner (1992)proposed a deterministic approach for the combinedestimation of model parameters and source character-istics. Bagtzoglou, Doughery and Thompson (1992)used backward location probabilities to identify con-taminant sources. Probability maps were then devel-oped using a random walk method by reversing the flowfield and assuming that dispersion was constant.The foundational advection/dispersion equation

used in traditional groundwater models is also usedfor backward extrapolation modeling (Wilson andLinderfelt, 1991; Wilson and Rao, 1992; Liu, 1995).An example of one expression used to run an

advection-dispersion one-dimensional model backwardin time (similar to backward particle and fronttracking) (Shafer, 1987) is described by Wilson andLiu (1995) by:

@g=@t D@2g=@x2 V@g=@x 28where g is a probability density or cumulativedistribution function of interest, D is the dispersioncoecient, t is time and x is the upgradient distance tothe source. The authors hypothesized that by selectingappropriate initial and boundary conditions, equation(28) can solve the desired probability function to createtravel time and location probability maps when thelocation of the release is unknown. Neupauer andWilson (1999) describe the solutions for source andtime probabilities for a pumping well as:

@fx=@t D@2fx=@x2 n@fx=@x 29where fx! 0 as x! 1, and

nfx D@fx=@x 0 at x 0 and fxx; 0 dx xd

where t is backward time (t td t, where td is thedetection time), xd is the detection location at apumping well (xd 0), D is the dispersion coecient,n is the groundwater velocity, fx is the locationprobability, and d is the Dirac delta function. Thesolution for equation (29) provides the backwardprobability for a detection at xd 0, as given byNeupauer and Wilson (1999) by:

fxx; t 1= pp texpfx nt2=4Dtg n=2Dexp fnx=Dgerfcx nt= 4p Dt: 30

For a contaminant parcel removed at the pumpingwell (xd 0), equation (30) describes the probabilitydensity function of the location of the contaminantparcel at time t before it was observed at the pumpingwell. The backward model for the travel timeprobability map is then (Wilson and Liu, 1995):

@ft=@t D@2ft=@x2 n@ft=@x 31where ft ! 0 as x! 1, and nft D@fr=@x ndt at x 0, and ft(x, 0) 0, and where D and nare constant and the pumping well is assumed to be atxd 0. The solution to equation (31) is the backward-in-time time probability from location x to a detectionat xd 0 is (Wilson and Liu, 1995):

ftx; t 1= pp texpfx nt2=4Dtg n2=2Dexpfnx=Dgerfc x nt= 4p Dt: 32

For a contaminant detected at the pumping well,equation (32) describes the probability density functionof the travel time from an upgradient source location(x) to the pumping well at xd 0.In cases where light nonaqueous phase liquids

(LNAPLs) are of interest, models using the directestimate and nonlinear parameter estimation method

Review of Environmental Forensic Techniques 189have been proposed. The direct estimation approachinvolves the estimation of a release date from a single

measurmethodknown.tion (eknown.is that orequireThe n

the simreleaseparamemeasursteps. Mtheir coregressiIf an

of a phand parameters are needed (Parker and Lenhard, 1990;Butcher and Gauthier, 1994). The solution of theinverseinvolveto coparamevariablthree-ping therepreseassumeand/orLNAPLestimathundre(less thacontinuincludeviscositfor thesoil texmeasurmentsobtainecan, thweatherother c

Challen

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190 R.problem for identifying NAPL locations estimating dissolution parameters in additionnventional flow and transport modelingters (Khachikian and Harmon, 2000). A keye for phase-separate models is whether thehase (air, water, and NAPL) expression describ-pressure distributions between these fluids is

ntative of the site. LNAPL models normallythat the phase-separate liquid is continuousconnected. If this assumption is incorrect, thevolume in the subsurface can be over-

ed. If monitoring wells are separated byds of feet and the LNAPL thickness is thinn a foot), the possibility that the LNAPL is notous is highly probable. Other challengesthe accuracy of LNAPL or DNAPL fluidy values, the selected interfacial tension valuesdierent liquids, the values used to represent theture, and the accuracy of any phase-separateements. Fluid viscosity and density measure-are rarely measured and are more commonlyd from the literature. These published numberserefore, deviate from the field values due toing and/or commingling of the LNAPL withompounds.

ges to inverse models

cessful review of an inverse model includes anof model parameters and conceptual frame-

Examples of model parameters incorporatedany inverse models and which represental sources of uncertainty include:

accuracy of the eective porosity value(s)ted;onsistency of the