12 Chlorinated Solvents -...

12 Chlorinated Solvents

Robert D. Morrison, Brian L. Murphy, and Richard E. Doherty

Contents12.1 INTRODUCTION 26012.2 CHLORINATED SOLVENT CHEMISTRY 26012.3 DEGRADATION REACTIONS

AND PATHWAYS 26112.4 ANALYTICAL METHODS 26212.5 HISTORICAL SOURCES

AND COMPOSITION OFCHLORINATED SOLVENTS 262

12.6 FORENSIC TECHNIQUES 267REFERENCES 273

260 CHLORINATED SOLVENTS

12.1 INTRODUCTION

Chlorinated solvents are one of the contaminants most fre-quently encountered in environmental forensic investiga-tions. This chapter provides a presentation of the chemistryof the most commonly used chlorinated solvents, degrada-tion pathways for these compounds, a historical perspectiveregarding their use and production, and forensic techniquesavailable for source identification and age dating. While thereare many chlorinated solvents of interest encountered inforensic investigations, the focus of this chapter is on the fol-lowing five:

• Trichloroethylene (TCE);• Perchloroethylene (PCE or tetrachloroethylene);• 1,1,1-Trichloroethane (TCA);• Carbon tetrachloride (CT); and• Methylene chloride (MC).

The detection of additives that are associated with a partic-ular compound or time period, isotopic analysis and molarratio analysis of the degradation products of these chlo-rinated solvents are the primary forensic techniques pre-sented in this chapter.

12.2 CHLORINATED SOLVENT CHEMISTRY

The chemistry and physical characteristics of chlorinatedsolvents are well understood and documented in the lit-erature (Bouwer and McCarty, 1983; Wilson and Wilson,1984; Kleopfer et al., 1985; Barrio-Lage et al., 1986; Fogelet al., 1986; Egli et al., 1987; Vogel et al., 1987; Fathepureet al., 1987; Vogel and McCarty, 1987; Janssen et al.,

Table 12.2.1 Summary of Physical and Chemical Properties of Selected Chlorinated Solvents at 25 �C

CompoundMolecular

Weight

VaporPressure(p �, torr)

Solubility(S, mg/L)

Henry’s constant(H, atm-m3/mol)

RelativeVapor

DensityBoiling

Point (�C)

dichloromethane 84�9 415 20000 0�00212 2�05 41chloroform 119�4 194 8000 0�00358 1�80 62bromodichloromethane 163�8 64�2 4500 0�00206 1�39 90dibromochloromethane 208�3 17 4000 0�00115 1�14 119trichlorofluoromethane 137�4 796 1100 0�0888 4�91 23�8carbon tetrachloride 153�8 109 825 0�0298 1�62 76�71,1-dichloroethane 99 221 5100 0�00543 1�70 57�31,2-dichloroethane 99 82�1 8500 0�0015 1�26 83�51,1,1-trichloroethane 133�4 124�6 1300 0�0167 1�591,1,2-trichloroethane 133�4 24�4 4400 0�00108 1�12 113�71,1,2,2-tetrachloroethane 167�9 6�36 2900 0�000459 1�04 146�41,1-dichloroethylene 97 603 3350 0�0255 2�86 31�9cis-1,2-dichloroethylene 97 205 3500 0�00374 1�63 60trans-1,2-dichloroethylene 97 315 6300 0�00916 1�97 48trichloethylene (TCE) 131�5 75 1100 0�00937 1�35 86�7tetrachloroethylene (PCE) 165�8 18�9 200 0�0174 1�12 121�41,2-dichloropropane 113 52�3 2800 0�00262 1�20 96�8trans-1,

3-dichloropropylene110 34 2800 0�0013 1�10 112

bis(chloro)methylether 115 30 22000 0�00021 1�09 104bis(2-chloroethyl)ether 143 1�11 10200 0�00013 1�004 178bis(2-chloroisopropyl)ether 171 0�73 1700 0�00011 1�003 1892-chloroethylvinylether 106�6 34�3 15000 0�00025 1�010 108chlorobenzene 112�6 11�7 500 0�00390 1�04 132o-dichlorobenzene 147 1�39 140 0�00198 1�01 179m-dichlorobenzene 147 2�25 119 0�00325 1�01 172

1988; McCarty, 1994; Benker et al., 1994; Morrison et al.,1998; Morrison, 1999). The intent of this section is not topresent a detailed summary of this information but rather toshare physical and chemical properties of chlorinated sol-vents of most direct application in environmental forensicinvestigations.

The most commonly encountered chlorinated solvents inenvironmental forensic investigations are TCE, TCA, PCE,CT, and MC. Physical and chemical properties of these sol-vents are summarized in Table 12.2.1. The high solubility ofmost chlorinated solvents is of special interest given theirpreference to dissolve into soil pore water and groundwater.Similarly, the vapor pressure of these solvents is importantrelative to their susceptibility to be used as a tracer in soilgas investigations. The last column in Table 12.2.1 summa-rizes information on the boiling points of the solvents. Thisis particularly important relative to the suitability of a partic-ular solvent for vapor degreasing. Methylene chloride andChlorofluorocarbon-113 (CFC-113) have low boiling pointswhile PCE’s boiling point is the highest. While other chlo-rinated solvents are volatilized in a degreaser using hotwater pipes, PCE requires steam pipes or an electric heater.The maximum recommended boiling temperatures for PCE,TCE, TCA, and MC in a distillation still, for example, are270, 210, 200, and 125 �F, respectively (Ethyl Corporation,no date). The third column shows the vapor pressure foreach of the solvents. This property is also important in vapordegreasing where the solvents are heated to their boilingpoints. The higher the boiling point, the higher the energyneeded to boil the solvents. The higher the latent heat ofvaporization, the higher the energy needed to keep the sol-vent at its boiling point.

DEGRADATION REACTIONS AND PATHWAYS 261

12.3 DEGRADATION REACTIONS AND PATHWAYS

Degradation pathways of chlorinated solvents are importantin understanding the fate and transport of these chemi-cals in the subsurface and form the basis for the use ofthese relationships in forensic investigations. The degrada-tion of chlorinated solvents in soil and groundwater occursby chemical (abiotic) and microbial (biotic) processes andis well understood (see Figures 12.3.1 and 12.3.2).

Numerous biochemical and abiotic reactions arepotentially involved in chlorinated solvent degradation.Details regarding dechlorination mechanisms leading, forexample, to the sequential degradation of TCE to cis-1,2-dichloroethylene (cis-1,2-DCE) are often treated in a non-specific manner. Four types of reactions are commonlyrecognized as responsible for organic compound degra-dation; substitution, dehydrohalogenation, oxidation, andreduction reactions (Schwarzenbach et al., 1985).

Dehydrohalogenation reactions involve the elimination ofHCl and the creation of a C=C double bond in place ofa single bond �C C�. Oxidation processes may involve anumber of mechanisms that add oxygen to the structureof the organic molecule. Epoxidation, where the C=C bondis replaced by a single bond and a mutually bonded oxy-gen, is possibly the most common. The potential for oxida-tion decreases as the degree of chlorination (number of Clatoms on the molecule) increases. Thus only the simplerdegradation products (single and double chlorine molecules

such as 1,2-DCE and vinyl chloride) can effectively be oxi-dized.

Reduction reactions typically involve hydrogenolysis(where a C Cl bond is broken and Cl− is replaced by H+)and dihalo-elimination (where two chlorines are removed,and a C=C bond is produced from a C C bond). Abioticand biotic reductive dechlorination is reported to occur formany chlorinated solvents. Biotically mediated processescomprise the focus of degradation under reduced condi-tions. Reduction reactions are more favored to occur forthe more highly chlorinated compounds such as TCE andPCE that contain three and four chlorine atoms, respectively(McCarty and Semprini, 1994).

The most commonly recognized degradation pathwaysfor chlorinated solvents are those that occur biotically underanaerobic conditions and for the less chlorinated com-pounds (one or two chlorine atoms) under aerobic condi-tions. The exception is TCA, which degrades abiotically byhydrolysis with time scales of interest under typical aquiferconditions (Vogel et al., 1987; McCarty, 1993). The com-pounds are presented vertically in Figures 12.3.1 for PCEand TCE and 12.3.2 for TCA as a function of the degree ofchlorination for each compound.

The degradation reactions and ultimately the efficiency ofdegradation processes vary as a function of the environmen-tal conditions and reaction types. First-order rate processesare used to describe rates of transformation of inorganicchemical species in aqueous solution. This approach is often

Figure 12.3.1 Degradation pathways for PCE and TCE.


Figure 12.3.2 Degradation pathways for 1,1,1-TCA.

used as a first approximation to evaluate biotic and abioticdegradation rates. The rate of change of a chemical in solu-tion is given in terms of half-lives. The concentration variesas a (negative exponential) function of e−kt , where t is theelapsed time and k is the rate constant. This degradationrate process is easily calculated and is commonly used incontaminant fate and transport models.

Temperature effects are significant in biotic and abioticprocesses. Metabolic and enzymatic reaction rates that drivebiotic degradation processes generally increase with tem-perature. Using TCA as a case example for abiotic degra-dation, the abiotic transformation rate of TCA to 1,1-DCE(CH3CCl3=> CH2=CCl2 +H+ +Cl−; forming approximately20% of the degradation product) can be estimated as a func-tion of temperature (McCarty, 1993). At 10 �C, the degra-dation half-life of TCA is about 12 years. The observedhalf-life decreases to 4.9 years at 15 �, and to 0.95 yearsat 20 � (Note that acetic acid, where CH3CCl3 + 2H2O=>CH3COOH + 3H+ + 3Cl−, forms approximately 80% of thedegradation product.). These data suggest that extrapola-tion of data between sites or experiments must carefullyconsider temperature effects.

12.4 ANALYTICAL METHODS

Analytical methodology for measuring the concentration ofchlorinated solvents is mature and well developed. In theUnited States, volatile organic compounds are analyzed viaEPA Standard Method 8260B/624. For drinking water sam-ples, EPA Standard Method 524.4 is employed. Both meth-ods rely upon gas chromatography.

12.5 HISTORICAL SOURCES AND COMPOSITIONOF CHLORINATED SOLVENTS

Chlorinated solvents were used in a wide variety of 20th-century industries. Most noteworthy in terms of volumewere the aerospace, military, metal-working, and dry-cleaning industries. The popularity of these solvents in theseindustries derived from their low flammability and reactivity,ease of evaporation, and strong dissolving power.

Potential sources of releases to the environment variedover the course of time for each of the solvents. For exam-ple, in dry cleaning, CT was the first solvent to be used,followed by TCE and finally PCE (Morrison, 2003). In metalcleaning (“cold cleaning”) and vapor degreasing, TCE wasinitially popular, but was eclipsed in the 1970s by TCA,which became the dominant solvent for this purpose (Wolf,1997). The trend reversed when TCA was phased out inthe 1990s, resulting in a resurgence in TCE use. The evolu-tion of uses of CT, TCE, PCE, and TCA in the 20th-centuryUnited States was summarized in 2000 in two review articlesby Richard Doherty (Doherty, 2000a, b).

During the first 60 years of the 20th century, the useof these solvents was influenced primarily by economicconditions and wartime demand. Increased demand dur-ing wartime years was generally followed by a period ofpost-war oversupply. During the latter 40 years, environ-mental regulation played an increasing role. Federal regu-lations and standards arising from the 1977 Clean WaterAct, the 1980 Resource Conservation and Recovery Act, andthe 1990 Clean Air Act Amendments directly affected theuse and handling of chlorinated solvents. The Clean AirAct facilitated the United States’ implementation of the 1990

HISTORICAL SOURCES AND COMPOSITION OF CHLORINATED SOLVENTS 263

amendments to the Montreal Protocol, which set a schedulefor an international ban of most uses of TCA and CT.

In pure form and in appropriate containers, chlorinated sol-vents can be stored for extended periods without degradation.However, in commercial use, the solvents are exposed to anumber of environmental factors that can cause degradationand/or diminish solvent effectiveness. These factors includeexposure to oxygen, light, metals/metal salts, water, hightemperatures, strong bases, and oxidizing agents (SolvayChemicals, 2002). The degradation of chlorinated solventsproduces hydrogen chloride (HCl), which degrades metalsurfaces, sometimes yielding products that initiate furtherdegradation. Additives were utilized with each of the solventsto varying degrees to address these issues.

Historical information regarding the five most commonlyencountered chlorinated solvents in forensic investigations(CT, PCE, TCE, TCA, and MC) is presented below.

12.5.1 Carbon Tetrachloride (CT)Carbon tetrachloride was the first of the five chlorinated sol-vents to come into general use. Production of commercialquantities in Europe began in approximately 1900 or earlier,and in the United States between 1905 and 1908. AlthoughCT was used for a wide variety of commercial and indus-trial applications, its major uses in terms of quantity wereas a cleaner, and as a raw material in the manufacture ofother chemicals, particularly CFCs. The latter constitutedthe largest single use of CT since World War II. Other CTapplications of note included it’s use in fire extinguishers,dry cleaning (before the use of PCE became prevalent),grain fumigation, military smokescreens, and as a solventfor lacquers (Doherty, 2000a).

Although CT production in the United States peaked inthe 1970s, roughly 80–95% of the CT produced during thattime was consumed in the manufacture of CFC-11 and CFC-12. Increasing awareness of toxicity and bans on specificuses were factors in the production decline that began in themid-1970s and continued in the following decades. Carbontetrachloride was banned in consumer products in 1970, inaerosol products in 1978, and in grain fumigation in 1985(Holbrook, 1991; NIH, 1999).

The earliest commercial-scale process for manufactur-ing CT employed chlorination of carbon disulfide. As aresult, small quantities of carbon disulfide (about 1 partper million (ppm) in technical grades and 100 ppm in com-mercial grades) can be found in CT produced by thismethod. Trace concentrations of bromine, chloroform, andhydrochloric acid may also be present in CT (Brallier, 1949;Holbrook, 1991).

Beginning in the 1950s, production of CT using thepyrolytic chlorination of methane or propane (also knownas chlorinolysis) began. This method soon became predom-inant; however, the carbon disulfide process continued tobe used commercially into the 1990s (Doherty, 2000a).

In metal cleaning applications, CT was replaced by TCEand other compounds due to its high toxicity, and becauseit tended to leave metal surfaces susceptible to corrosion(Brallier, 1949). Carbon tetrachloride was usually shipped ingalvanized tin or lead-lined containers for this reason. Lowconcentrations of corrosion inhibitors were used with someCT formulations in an effort to address this problem. Chem-icals used included diphenylamine (0.34–1%), ethyl acetate(to protect copper), alkyl cyanamides, ethyl cyanide (up to1%), and thiocarbamide (DeShon, 1979; Holbrook, 1991).However, other literature state that commercial grades ofCT rarely contained inhibitors (DeForest, 1979). Despitethese inconsistencies in the literature, most researchersagree that concentrations of additives in CT formulationswere generally low relative to other chlorinated solvents,such as TCE and TCA.

12.5.2 Tetrachloroethylene (PCE)Most commonly known for its widespread use in dry-cleaning, PCE has several other important uses. PCE wasused for metal cleaning and degreasing, particularly forcleaning aluminum prior to the development of stabilizedTCA formulations, and for the removal of wax and resinresidues. Tetrachloroethylene is also used in cleaning small,low-mass parts because the condensed solvent contact time,before the part reaches the vapor temperature, is longerthan with other solvents. Other uses included automotivebrake cleaning, rubber dissolution, paint removal, sulfurrecovery, printing ink bleeding, soot removal, and catalystregeneration (Lowenheim and Moran, 1975). PCE was usedin various textile operations as a scouring solvent, a carriermedium, and for spot removal. The primary use of PCE after1996 was in the production of fluorinated compounds suchas CFC-113 and HFC-134a.

By the late 1940s, PCE had surpassed CT as the pre-dominant non-petroleum dry-cleaning solvent. Peak yearsfor PCE production in the United States ranged from thelate 1960s to the early 1980s, when approximately 600–700 million pounds per year were produced. The effects ofenvironmental regulation and significant improvements inthe dry-cleaning process resulted in an overall decreaseddemand for PCE by the 1980s (Doherty, 2000a). In manycountries, this decrease in demand resulted in the cessa-tion of PCE manufacturing. In Australia, for example, PCEmanufacturing ceased in 1991 (NICNAS, 2001). In WesternEurope, the production and sales of PCE more than halvedbetween 1986 and 1994 (Linn, 2002). In the United States,the use of PCE for dry cleaning in 2000 has decreased toabout one-sixth of the levels of the 1970s.

Tetrachloroethylene production processes are capable ofproducing product of 99.9% purity (Lowenheim and Moran,1975; Gerhartz, 1986). The primary method of PCE pro-duction prior to the 1970s involved the chlorination ofacetylene to produce both PCE and TCE. Subsequent pro-duction methods could produce TCE as a co-product (e.g.,high-temperature chlorination, oxychlorination) or CT asa co-product (chlorinolysis) (Hickman, 1991). Ethanol is areported impurity in PCE.

Because PCE is a relatively stable molecule, small con-centrations of stabilizer additives are needed relative toother chlorinated solvents. Reported antioxidant (i.e., amineor phenolic compound) concentrations for metal-cleaninggrades range from 50 to 200 ppm. Concentrations of acidacceptors, such as an epoxide, for PCE range from 0.2 to0.7% (Archer and Stevens, 1977).

Tetrachloroethylene used for dry cleaning is usually ofhigh purity. In the 1960s, Dow Chemical offered a dry-cleaning grade of PCE (DOWPER-C-S), which reportedlycontained six additives. These included a redepositionagent, a water-soluble detergent, a corrosion inhibitor, ananti-static compound, “hand agent” additives, and a scav-enger for fatty acid control. Dow tested the product in 1963,and brought it to market in approximately 1967 (Chemicaland Engineering News, 1963; Dow Chemical, 1970, 1971a, b,1973). The intent of the product was to save the dry cleanerfrom the effort of pre-mixing PCE with detergents, particu-larly for use in coin-operated dry-cleaning machines, whichwere becoming popular at that time.

Tetrachloroethylene used for vapor degreasing typicallyhad a higher concentration of additives than mostdry-cleaning grades (Von Grote, 2003; Dow Chemical,2005a). Classes of chemicals used include acid acceptors,antioxidants, and ultraviolet (UV) light stabilizers. Acidacceptors were only required for PCE when used in high-temperature or other “stressful” applications (Archer, 1996).Alkylamines and other hydrocarbons were added to early


PCE formulations; later stabilizers included morpholinederivatives (Gerhartz, 1986). Epoxides, esters, and phenolshave also been used as PCE additives (Mohr, 2001; Morri-son, 2003) (see Table 12.5.1).

12.5.3 Trichloroethylene (TCE)Trichloroethylene’s solvent properties resulted in itswidespread use in commerce and industry. TCE has beenused in the electronics, defense, chemical, rail, adhesive,automotive, boat, textile, food processing, and dry-cleaningindustries (ASTR, 1997). It is an excellent solvent because ofits aggressive action on oils, greases, waxes, tars, gums, andcertain polymers, and has historically been the major sol-vent used in industrial vapor degreasing and cleaning appli-cations (Wood, 1982; ASM, 1996). Trichloroethylene hasalso been used in inks, paints, paint removers, adhesives,fire extinguishers, lubricants, pesticides, polishes, pipe anddrain cleaners, medical and dental anesthetics, and manyother industrial and consumer products (Doherty, 2000b).

Commercial-scale TCE production in the United Statesbegan in approximately 1921, and increased as it becamewidely used in dry cleaning and vapor degreasing. Usageof TCE in dry cleaning decreased in the early 1950s afterit was found to degrade cellulose acetate dyes. By 1952, itwas estimated that 92% of TCE was used in vapor degreas-ing (Chemical Week, 1953). Production grew steadily until1970, when annual production peaked at approximately 600million pounds (Doherty, 2000b). The decline in productionthat began in 1970 was the result of increasing evidenceof toxicity, economic factors, and increased environmen-tal regulation. Its use as a solvent experienced a reboundin the 1990s when it was listed as a recommended sub-stitute for other solvents (such as TCA) banned underthe Montreal Protocol and the Clean Air Act Amendments(Kirschner, 1994). In 1991, Sweden issued an ordinance thatbanned the sale, transfer or use of chemical products con-taining TCE. This ordinance became effective for consumersand industry in 1993 and 1996, respectively (NICNAS, 2000).Austria and Switzerland also have regulations banning cer-tain chlorinated solvent applications (KEMI, 1995).

Prior to 1950, TCE was produced almost exclusively fromacetylene. By 1978, the acetylene production method wasno longer in commercial-scale use in the United States. Dur-ing the 1960s and 1970s, TCE was increasingly produced

from ethylene or 1,2-dichloroethane (1,2-DCA) using chlo-rination processes. Different countries have unique produc-tion chronological histories; in Australia, for example, TCEwas manufactured by ICI at their Botany chemicals facilityin Sydney between 1948 and 1977 (ICI Botany Operations,1996).

Unstabilized TCE is typically greater than 99% pure(Gerhartz, 1986); however, it is particularly vulnerable tooxidation when exposed to air, light, or heat. Stabilizingcompounds were used in TCE to prevent chemical break-down. Degradation can occur especially rapidly in the pres-ence of aluminum, producing significant quantities of HClgas. Typical stabilizer formulations included an acid accep-tor, a metal stabilizer, and/or an antioxidant. These addi-tives typically comprised from 0.1 to 0.5% of the solvent, butconcentrations reportedly ranged as high as 2%. In vapordegreasing grades, concentrations at the higher end of therange were typical, and additional compounds might beadded to enhance thermal stability.

A variety of chemicals have been used as TCE addi-tives, including alcohols, amines, ethers, esters, epoxides,substituted phenols, and heterocyclic nitrogen compounds.The earliest stabilizers for PCE and TCE were gasolineand other unsaturated hydrocarbons (Shepherd, 1962).Until 1954, the most commonly used acid acceptors inTCE were amines, including trimethylamine, triethylamine,triethanolamine, aniline, and diisopropylamine (ChemicalEngineering, 1961). One source cites the typical trimethy-lamine concentrations as 20 ppm by weight (Lowenheimand Moran, 1975). Kircher reported that typical concentra-tions ranged from 10 to 100 ppm (Kircher, 1957). In themid-1950s, amines began to be replaced by non-alkaline for-mulations, particularly a pyrrole-based, six-to-seven compo-nent mixture developed by DuPont. Metal stabilizers usedwith TCE included epoxides such as 1,2-butylene oxideand epichlorohydrin. The use of the latter was discontin-ued in the 1980s due to its toxicity (Mertens, 1991). Addi-tives for thermal stability primarily included cyclohexene,diisobutylene, and amylene, although many others wereused (Shepherd, 1962). Table 12.5.2 lists acid inhibitors,metal inhibitors, antioxidants, and light inhibitors associatedwith TCE (Hardie, 1964; Mohr, 2001; Morrison, 2003).

Solvent stabilizers and inhibitors for TCE can be found atsignificantly different concentrations in still bottom residue“muck” than in virgin TCE. Stabilizers were found to be

Table 12.5.1 Tetrachloroethylene Additives (after Morrison, 2000c; Mohr, 2001)

Acid Inhibitors Metal Inhibitors Light Inhibitors Antioxidants

Acetylenic alcohols Alcohols Amines Acetylene ethersAcetylenic carbinols Aromatic hydrocarbons Cyanide PhenolsAcetylenic esters Hydroxyl aromatic

compoundsAlcoholsNitriles

Pyrrole

Aliphatic amines

Cyclic trimers

Organo-metalliccompounds

Thiocyanates

Aliphatic monohydricalcohols

EstersLactone

Amides

Oxazoles

Amines

Oximes

Azo aromaticcompounds

Sulfones

Epoxides

Sulfoxide

Hydroxyl aromaticcompounds

KetonesNitroso compoundsPyridines

HISTORICAL SOURCES AND COMPOSITION OF CHLORINATED SOLVENTS 265

Table 12.5.2 Trichloroethylene Additives (after Morrison, 2000c)

Acid Inhibitors Metal Inhibitors Antioxidant Light Inhibitor

Acetylenic alcohols Alcohols Amides Aromatic benzene nucleiAlcohols Amides Amines BoranesAliphatic amines Amines Aromatic carboxylic acids EthersAliphatic monohydric

alcoholsAromatic hydrocarbons GuanidineComplex ethers and

oxides

Alkyl pyrrolesHydroxyl-aromatic compounds

AlkaloidsAryl stibine

Alky Halides CyanideBoranes Organo-metallic compounds

Amines Cyclic EthanesButylhydroxyanisole

Azines Cyclic trimersPhenols

Azirdines EpoxidesPyridines

Azo-aromatic EstersPyrrole

compounds EthersThiocyanates

Epoxides KetonesEssential oils OlefinsHydroxyl-aromatic Peroxides

compounds PyridinesNitroso compounds OxazolesOlefins OxazolinesOrganic Substitute OximesNH4 hydroxides SulfonesOxirane SulfoxidePhenols ThiophenePyridinesPyrroleQuatenaryAmmonium

retained in still bottoms in excess of 35% of their con-centration in original TCE (Joshi et al., 1989). For vapordegreasers, as much as 50% of the still bottom residue issolvent (United States Environmental Protection Agency,1979). The significance of this information is that while theoriginal concentrations of these stabilizers in the virgin TCEmay be less than 1%, when accumulated in still bottoms andthen released into the environment, they may be presentat significantly elevated concentrations. Another example ofdifferent concentrations of TCE stabilizer concentrations asa function of use is summarized in Table 12.5.3 for butyleneoxide, epichlorohydrin, ethyl acetate, and methyl pyrrole(Hardie, 1964; Mohr, 2001).

12.5.4 1,1,1-Trichloroethane (TCA)The primary applications of TCA (methyl chloroform) werein cleaning and degreasing, where it served as a less toxicreplacement for TCE, PCE, and other solvents. Similar toTCE, TCA was used in a variety of industries and purposes.The aircraft, automotive, electronic, and missile industrieswere significant users of TCA. Among the many products

that included TCA were pesticides, drain cleaners, aerosolpropellants, and carpet glue.

Although occurred TCA production in the United States inthe mid-1930s, the chemical did not see significant commer-cial use as an end product until the mid-1950s (HalogenatedSolvents Industry Alliance, 1994). Its acceptance was tied tothe development of suitable stabilizer formulations. Its pro-duction increased steadily throughout the 1960s and 1970s,and first surpassed the production of TCE in 1973 (Doherty,2000b). Production peaked in the mid-1980s and then beganto decrease due to increased environmental regulation anda heightened awareness of environmental impacts. BecauseTCA has a lower boiling point than either TCE or PCE,it had special applications for cleaning items such as com-puter boards and electric components, all of which can bedamaged by high temperatures (Soble, 1979; Warner andMertens, 1991). In 1989, the industrial use of TCA was sum-marized as: metal degreasing (32%); cold cleaning (19%);aerosols (11%); adhesives (9%); chemical intermediate (9%);electronics (7%); coatings and inks (6%); textiles (3%); andmiscellaneous (4%). The 1990s marked the beginning ofthe end of TCA’s use as a solvent, as its ozone-depleting

Table 12.5.3 Stabilizer Concentrations in New and Spent TCE (after Mohr, 2001)

SampleDescription Stabilizer Concentration (Weight Fraction)

SampleButylene Oxide

�×103�Epichlorohydrin

�×103�Ethyl Acetate

�×103�Methyl Pyrrole

�×104�

New TCE 1�64 1�66 3�46 1�59Spent TCE 0�685 1�69 2�85 2�18TCE Distillate 0�718 1�61 2�58 1�66Carbon Adsorbed TCE 0�44 1�31 2�65 0�90


potential caused it to be phased out under the 1990-amendedMontreal Protocol and the 1990 Clean Air Act Amendments.Most emissive uses in the United States were phased outby the end of 1995.

The commercialization of TCA as a metal-cleaning andmetal-degreasing solvent was hindered by the lack of effec-tive stabilizers. Prior to the mid-1960s, TCA was foundto be unacceptable for use in heated degreasing due tothe absence of these inhibitors (solvents progressivelydeteriorate due to exposure to ultra-violet light and heat).Uninhibited TCA in contact with aluminum forms aluminumchloride, 2,2,3,3-tetrachlorobutane, 1,1-DCE, HCl, and mag-nesium. Improperly stabilized TCA can also decomposein the presence of magnesium. The use of TCA in vapordegreasing involves condensing the vapors with cold waterpipes or a refrigeration unit near the top of the tank sothat the liquid TCA is returned to the solvent bath. In theprocess water is condensed from the air. Uninhibited TCAhydrolyzes when boiled with water to produce hydrochloricand acetic acid (Manufacturing Chemists’ Association, 1965;United States Environmental Protection Agency, 1979).

As a result, unstabilized TCA could not be used formany metal-cleaning applications, particularly in the high-temperature environment of a vapor degreaser. Whenheated to a range of 360–440 �C, TCA decomposes to 1,1-DCE and HCl. Stabilizers used with TCE were found tobe only partially effective with TCA; therefore, significantefforts were expended by chemical companies to find newstabilizers for use with TCA. Until suitable stabilizing for-mulations were developed, TCA use with metals was largelylimited to cold cleaning (Bachtel, 1958; United States Army,1978; Jordan, 1979). In the 1960s Dow Chemical Companymarketed a solvent based on TCA that was specificallydesigned for spray-cleaning railroad equipment (Chemicaland Engineering News, 1962).

Similar to TCE, stabilizer formulations used for TCAtypically included an acid acceptor and a metal stabilizer.However, unlike TCE and PCE, an antioxidant was not typ-ically needed due to TCA’s stability to oxidation. However,antioxidants were sometimes added to TCA (particularlyvapor-degreasing grades) to prevent degradation of otheradditives (Jordan, 1979).

Due to TCA’s greater reactivity with metals, the over-all concentration of additives was typically higher in TCAthan in TCE. Concentrations of stabilizing chemicals rangedbetween 3 and 8% (Lowenheim and Moran, 1975), butwere frequently in the range of 4–6%. Typical concen-trations in United States vapor-degreasing grades werereported as 2–3.5% 1,4-dioxane, 1–2% sec-butanol, 1% 1,3-dioxalane, 0.4–0.7% nitromethane, and 0.5–0.8% 1,2-butyleneoxide (Archer, 1984; United Nations Industrial DevelopmentOrganization, 1994).

Additive formulations varied between manufacturers andbetween grades produced by the same manufacturer.Typically, epoxides, ethers, amines, and alcohols wereused as acid acceptors, and nitro- and cyano-organo com-pounds were used as metal stabilizers. A wide varietyof chemicals has been reportedly used in TCA sta-bilizer formulations, including 1,4-dioxane; 1,3-dioxolane(synonyms include 1,3-dioxolane, glycol formal, 1,3-dioxole, dioxolane, Glycol methylene ether, dihydroethy-lene, glycol formal, and formal glycol); 1,2-butylene oxide(synonyms include 1,2-dpoxybutane, EBU, propyl oxi-rane, epoxybutane, and 2-ethyloxirane); epichlorohydrin(synonyms include chloromethyloxirane, glycidyl chloride;chloropropylene oxide, glycerol, epichlorohydrin, 1,2-epoxy-3-chloropropane, 3-chloro-1,2-epoxypropane, gamma-chloropropylene oxide, 1-chloro-2,3-epoxypropane, and2,3-dpoxypropyl chloride); nitromethane (synonyms include

NMT and nitrocarbol); 1,2-epoxybutane; methyl ethylketone; n-methyl pyrrole; ethyl acetate; tetraethyl lead; n-methyl pyrrole; acrylonitrile; isopropyl alcohol; monohydricacetylenic alcohols; glycol diesters; nitriles; butyl alcohols;tetrahydrofuran (synonyms include THF, 1,4-epoxybutane,cyclotetra-methylene oxide, oxacyclopentane, and oxolane);morpholine; toluene; and dialkyl sulfoxides, sulfides, andsulfites (Irish, 1963; Lowenheim and Moran, 1975; Jordan,1979; Archer, 1982). Several hundred TCA stabilizer formu-lations have been patented worldwide (Snedecor, 1991).

Dow Chemical, the first and only major TCA manu-facturer in the United Sates until 1962, introduced theChlorothene brand of TCA in 1954 (USPTO). Chlorothenecontained inhibitors that allowed its use as an aerosolpropellant (Chemical Week, 1956), but it was not rec-ommended for use with aluminum (Barber, 1957). Inpatents issued in 1954 and 1955, Dow registered theuse of 1,4-dioxane (synonyms included DX, 1,4-diethylene-dioxide, diethylene oxide, p-dioxane, tetrahydro-1,4-dioxan,dioxyethylene-ether, and glycolethylene ether) and a non-primary alkonol, the first effective TCA stabilizer system foruse with aluminum (Bachtel, 1957). Chloroethene NU, intro-duced by Dow in May 1960, utilized the 1,4-dioxane-basedstabilizer system (Chemical and Engineering News, 1962).However, as of 1962, TCA was not recommended for usein vapor degreasers (ASTM, 1962). Further patents incor-porated modified formulations to address this need by thelater 1960s. Reported impurities found in unstabilized TCAinclude 1,2-DCA, 1,1-DCA, chloroform, CT, TCE, 1,1,2-TCA,and 1,1-DCE.

An important TCA stabilizer is 1,4 dioxane. 1,4-dioxanewas produced in the United States by Ferro Corporation,Dow Chemical (also imported 1,4-dioxane), and Stephancompany. Dow Chemical applied for a US patent to stabilizeTCA in 1954 with 1,4 dioxane at a concentration of 3.5%.In 1962, Dow Chemical applied for a patent that describedstabilizing TCA with a combination of 1–10% 1,4 dioxaneand 0.001–1% n-methyl pyrrole. The presence of dioxane inTCA is designed to prevent corrosion of aluminum, zinc,and iron surfaces by neutralizing hydrochloric acid. Approx-imately 90% of the 1,4-dioxane produced in 1985 was usedas a stabilizer for chlorinated solvents, especially for TCA(United States Environmental Protection Agency, 1995).While TCA is associated with other industrial and commer-cial uses (fumigants, an additive in antifreeze, cosmetics,a wetting and dispersion agent in textile processes, a sol-vent in paper manufacturing, in liquid scintillation counters,and an “inert” ingredient in herbicides [Roundup®, Pond-master®, Rattler®, Rodeo®]), its primary association is asa stabilizer in chlorinated solvents (Italia and Nunes, 1991;Scalia et al., 1992).

12.5.5 Methylene Chloride (MC)Methylene chloride (dichloromethane) did not become animportant industrial chemical until the years immediatelyafter World War II, when production increased fivefold. Thepeak years of production were the late 1970s and early1980s. A 1985 National Toxicology Program study indicatingthat MC caused cancer in mice was a factor in the decreaseddemand (NIOSH, 1986).

One of MC’s first uses was in paint strippers, and itremained the predominant use for many decades. Methy-lene chloride paint strippers remove many types of fin-ishes from a variety based of surfaces. Concentrations ofmethylene chloride is paint strippers range from 10 to90%. Methylene chloride was also used in metal cleaning,polyurethane foam production, in aerosol products formula-tion, adhesives (where it served as a replacement for TCA),and as an extraction solvent (e.g., in laboratories and in

FORENSIC TECHNIQUES 267

the pharmaceutical industry) (Archer, 1996; Dow Chemical,2005b). Methylene chloride was used in oven cleaners, tarremovers, refrigerants, and in the production of spices, beerhops, and decaffeinated coffee. Other uses included metaldegreasing and chemical processing. Due to its relativelylow boiling point, MC could be used for vapor degreasingof temperature-sensitive materials.

Unstabilized MC reacts with lighter metals, particularlyaluminum. Commercial grades of MC reportedly almostalways contained stabilizers (DeForest, 1979). Reported con-centrations of stabilizers in technical grades of methylenechloride range from 0.0001 to 1% (IPCS, 1987). Similarto other chlorinated solvents, grades of MC intended foruse in vapor degreasers typically have higher concentra-tions of stabilizing chemicals. Stabilizers included not onlycyclohexane, but propylene oxide (for aerosol and vapordegreasing formulations), methanol, ethanol, thymol, hydro-quinone, phenols and amines (e.g., tertiary butylamine)(DeForest, 1979; IPCS, 1987, 1996). Amylenes are reportedto be the most widely used stabilizers in laboratory-gradeMC (Hsu et al., 2005). Reported impurities in commercialgrades of MC include methyl chloride, chloroform, 1,1-DCA,and trans-1,2-DCA (IPCS, 1987).

Methylene chloride paint strippers can contain varyingconcentrations of other solvents such as methanol. Otherchemicals incorporated into MC paint strippers includeamines, acids, ammonium hydroxide, detergents, paraffinwax, and other alcohols.

12.6 FORENSIC TECHNIQUES

Forensic techniques commonly employed to discriminateand potentially age-date chlorinated solvent releases include(Morrison, 2000a,b,c; 2001; 2003; Morrison and Hener-oulle, 2000; Morrison, 2005):

• The presence of additives and/or chemical surrogatesassociated with chlorinated solvent releases;

• Isotopic analysis;• Ratio analysis using measured parent compound–

degradation product concentrations; and• Age-dating based on groundwater plume length.

While each environmental forensic investigation is unique,the use of these methods, or combinations thereof, oftenprovide the answers regarding the source and age of a chlo-rinated solvent release.

12.6.1 Solvent StabilizersAs discussed earlier in this chapter, chlorinated solventsmarketed for use in metal cleaning, degreasing, electron-ics, and textile cleaning require stabilizers (acid receptors,metal inhibitors, and antioxidants), so that the solventscan function as intended. Acid receptors are neutral (epox-ides) or slightly basic (amines) compounds that react withhydrochloric acid, which is commonly produced when sol-vents and oil decompose. Alcohol is normally formed in thisprocess (Archer, 1984). Metal inhibitors deactivate metalsurfaces and complex metal salts that might form. Antioxi-dants are added to solvents to reduce their potential to formoxidation products (Joshi et al., 1989).

Both TCE and TCA require metal inhibitors and acidacceptors whereas TCE requires only an oxidant. The TCEvapor-degreasing solvent marketed as NEU-TRI®, for exam-ple, is highly stabilized to prevent the accumulation of acid(Mohr, 2001). Tetrachloroethylene is considered to be rela-tively stable and has only minor amounts of acid inhibitorswhen used for degreasing and no metal inhibitors (Keil,1978). Methylene chloride is considered to be stable andrequires less than 0.1% of acid inhibitors.

The use and identification of solvent stabilizers to iden-tify the origin and age-date a contaminant plume requiresdetailed information regarding the original composition ofthe solvent released into the environment. In addition,unless concentrated in still bottom residue or other circum-stances, many of these additives are present in the originaladditive package at low concentrations, and are thereforedifficult to detect in an environmental sample. Since formu-lations of solvent packages vary depending on the countrywhere the solvent was synthesized, the origin of the originalsolvent is important (Morrison, 2000c).

An exception to these challenges is when a solvent sta-bilizer is recalcitrant to degradation and is present or isaccumulated at a detectable concentration. 1,4-dioxane, isan example of such a stabilizer that is available to tracethe release of 1,1,1-TCA (Nyer et al., 1991; Barone et al.,1992; Duncan et al., 2004). 1,4-dioxane can become signifi-cantly concentrated in recycled or used TCA (Mohr, 2001).Because 1,4-dioxane can accumulate during use and unlikeTCA, it does not readily degrade, it can act as an indicatorof a TCA spill even when TCA is not present, for exam-ple when TCA has degraded to a concentration below itsdetection limit. In addition, 1,4-dioxane is hardly retardedin groundwater, unlike TCA and its principal chlorinateddegradation products, hence the plume front from a TCAspill may only contain 1,4-dioxane. When using additives astracers, it is important to note that depending on the purityand whether the solvent was recycled, contaminants in thesolvent that are not intentionally part of the additive packagemay be present. For example, the analysis of contaminantsand additives in a TCA sample tested in 1989 contained thefollowing chemicals:

• butylene oxide (inhibitor);• 1,2-DCE (contaminant);• TCE (contaminant);• ethylene dichloride (contaminant);• 1,4-dioxane (inhibitor);• nitromethane (contaminant);• nitroethane (contaminant); and• 1,1,2-TCA (contaminant).

Surrogate chemicals used to associate the release of a chlo-rinated solvent also include chemicals other than additives.For example, in the United States PCE releases into the envi-ronment are often associated with dry-cleaning operations.The release of other compounds with chlorinated solvents(leather dyes, soaps, etc.) may provide evidence regardingthe age of a release if the surrogate compound was usedwithin a discrete time.

12.6.1.1 Accumulation of Less Volatile CompoundsDuring Degreasing Operations

When less volatile trace contaminants are present in adegreasing solvent they can accumulate during degreasingoperations and become enriched in the spent solvent. Thephenomenon is illustrated by calculation in this section forPCE as a minor contaminant in TCE. Even at sites wherePCE was ostensibly never used it may be found in the envi-ronment when spent TCE is discharged.

Perchloroethylene can be present in TCE, particularly indegreasing grades, because the two solvents are producedby the same process and are separated by fractional distilla-tion. Specifications at www.rbchemtrade.com indicate thatother chlorinated compounds comprise 3–4% of degreasing-grade TCE.

The boiling points of PCE and TCE are 121.2 and 87�2 �C,respectively. Trichloroethylene degreasers usually use hotwater below the boiling point of 100 �C. Because of this


Table 12.6.1 Example TCE Degreaser ParameterValues

Parameter Value

PCE Impurity Level 1%TCE Volume During Initial

Operation350 gallons

TCE Addition 55 gallons/weekCleaning Frequency 4 times/yearSpent Solvent Discharge Each

Cleaning100 gallons

difference in boiling points, TCE volatilizes into the air andis lost from the degreaser at a more rapid rate than PCE.

There are two steps to estimate the accumulation of PCEin TCE. First, calculate how much PCE enters the degreaseralong with the TCE and, second, estimate how much of thePCE volatilizes. For example, one can calculate the accu-mulation of PCE in a vapor degreaser using the propertiesshown in Table 12.6.1. The volume of spent solvent in Table12.6.1 that is discharged during cleaning is intended to rep-resent the volume below the hot water pipes used to boilthe TCE, less any solids that have collected. Trichloroethy-lene above the hot water pipes would typically be conservedby being boiled back to a separate tank. From Table 12.6.1the amount of TCE added between cleanings is found to be350+55×12 = 1015 gallons (solvent is not added just beforecleaning). The amount of PCE introduced to the degreaseris thus 10.15 gallons.

In estimating how much of the PCE would evaporate, itis necessary to estimate the vapor pressure of PCE in thedegreaser. Assuming that the solvent temperature duringoperation is approximately the TCE boiling point tempera-ture of 87�2 �C, the vapor pressure of PCE at this tempera-ture is about 0.34 atmospheres. This estimate is based onthe Clausius-Clapeyron equation:

ln Pv = constant �1

TB− 1

T�� (12.6.1)

where Pv is the vapor pressure in atmospheres and TBis the boiling point temperature in degrees Kelvin, whichis 394�35 K �121�2 �C�. Perry’s Chemical Engineer’s Hand-book (1999) gives the vapor pressure as 200 mg mercury or0.263 atmospheres at T = 352�95 K �79�8 �C�. From this wefind the constant in Equation 12.6.1 and find that the vaporpressure for PCE at the TCE boiling point temperature of360�35 K �87�2 �C� is about 0.34 atmospheres.

The loss of PCE is then estimated as a ratio to the TCEloss. Prior to cleaning the degreaser, 100 gallons of TCE areassumed to be present in the vapor degreaser and 200 gal-lons are present in a boil back tank.1 Thus the total TCEvolatilization loss is 1015−300 = 715 gallons.

The amount of PCE in the evaporated TCE would be7.15 gallons and the portion evaporated is estimated as theratio of the PCE to the TCE vapor pressure, the latter beingone atmosphere at the boiling point. Thus the estimatedloss is 0�34 × 7�15 = 2�43 gallons. This estimate assumesthat the only difference between PCE and TCE evaporation

1� This feature may not always be present, for example in degreasersequipped with solvent distillation. Its purpose is to preserve the bulkof the degreasing fluid for reuse while what remains in the maintank contains oil, grease, and dirt, and is disposed.

is the relative vapor pressures.2 Thus the amount of PCEpresent along with the 100 gallons of TCE in the main tankis the amount introduced minus the evaporative loss or10�15−2�43 = 7�72 gallons.

The percentage of PCE in the remaining 100 gallons ofTCE is about 7.7% indicating an enhancement from the origi-nal level of about 7.7 times. Note that in making this estimateTCE and PCE that was evaporated and recondensed intothe boil back tank has been treated like solvent that simplyevaporated into the air. If the 100 gallons of TCE/PCE isdischarged to the environment, the resulting PCE levels inthe environment may be high enough that it will appear thatPCE was also used as a degreasing fluid on-site, rather thana minor contaminant in the TCE.

This enhancement phenomenon also occurs with otherrelatively nonvolatile solvent additives, such as 1,4-dioxane.However, some additives are consumed during degreasingoperations and need to be considered.

12.6.2 Isotopic AnalysesCompound-specific isotope analysis (CSIA) represents amature methodology used in environmental forensic inves-tigations to (1) distinguish between different contaminantsources and/or (2) to demonstrate that biodegradation isoccurring (Hunkeler et al., 1997, 1999; Dayan et al., 1999;Sherwood Lollar et al., 1999; Sturchio et al., 1999; Dren-zek et al., 2002; Barth et al., 2004). These techniques havebeen successfully used to examine the biodegradation ofchlorinated solvents under either anaerobic (Slater et al.,2001) or aerobic (Barth et al., 2002) environments as evi-dence of natural attenuation and for source differentiationat field sites throughout the world (Hunkeler et al., 1999,2003; Dayan et al., 1999; Sturchio et al., 1998; Bloom et al.,2000; Slater et al., 2001; Bill et al., 2002; Mancini et al., 2002;Song et al., 2002; Kirkland et al., 2003; Hunkeler et al., 2004;Kloppmann, et al., 2005).

Heraty et al. in 1999 investigated the isotopic fractiona-tion of carbon and chlorine during the aerobic degradationof dichloromethane by MC8N, a gram-negative methy-lotropic organism related to the genera methylobacterium orochrobactrum, and found this technique a useful indicatorof microbial degradation. The current use of isotopic analy-ses for chlorinated solvents has focused upon the ability touse isotopic information as an indicator of biodegradationand to distinguish between different manufacturers and/orsources. The use of isotopic analyses as evidence of degrada-tion is especially intriguing not only for chlorinated solventsbut for other compounds such as methyl tertiary butyl ether(MTBE) and n-alkanes (Kelly et al., 1997; Stehmeier et al.,1999; Pearson and Eglinton, 2000; Gray et al., 2002; Pondet al., 2002; Reddy et al., 2002).

The use of isotopic analyses to distinguish betweensources of chlorinated solvent releases is premised on theassumption that a wide range of isotopic signatures existfor different manufacturers (Tanaka and Rye, 1991; Poul-son and Drever, 1999). This assumption implies that carbonisotopic fractionation is not expected to occur during syn-thesis unless there are incomplete reactions and/or recy-cling of by-products during the manufacturing process. Theprimary �13C variability is therefore likely associated withdifferences in the isotopic signature of the original carbonmaterials (Ertl et al., 1998). Halogenated compounds are

2� The relative rate of solvent loss is controlled by a “Henry’s Law”type constant, which for dilute solutions is equal to the ratio ofvapor pressure to solubility. However, assuming that PCE is infinitelymiscible in TCE, this reduces the relative rate of solvent loss beingapproximately proportional to the ratio of vapor pressures.


expected to exhibit a wide range of manufacturer-dependentisotopic signatures due to various chemical reactions, whichmay include dehydrochlorination or dehydrogenation reac-tions and production conditions (e.g., temperature differ-ences, catalysts used, engineering design, etc.) as well asuse of different feedstocks. The difference in bond strengthresults in chlorine isotope fractionation due to temperatureand pressure differences during synthesis (Tanaka and Rye,1991). The 37Cl isotope fraction in organic solvents is boundmore tightly to carbon than are 35Cl atoms (Bartholomewet al., 1954).

Isotopes used for distinguishing between different chlo-rinated solvent sources include 13C, 35Cl, and 37Cl (Clarkand Fritz, 1997). Van Warmerdam et al. (1995) examinedthe isotopic ratios for 13C/12C and 37Cl/35Cl for chlorinatedsolvents from four manufacturers. Beneteau et al. (1996,1999) continued this work using 13C and 37Cl for comparisonbetween two batches from the Van Warmerdam et al. studyand five pure-phase chlorinated solvents manufactured byDow Chemical and PPG. Of note is that significant differ-ences in the �13C of chlorinated solvents obtained from DowChemical and PPG and analyzed by van Warmerdam et al.in 1995 and by Beneteau et al. in 1999 exist. Of note isthat the �13C signatures of PCE, TCE, and TCA betweenbatches analyzed in 1995 and 1999 from the same manufac-turers (Dow Chemical and PPG) were found to be variable.For TCA, �37Cl values between batches supplied by PPGin 1995 and 1999 ranged from −2�90 to −0�36‰, respec-tively, suggesting that the use �37Cl values for TCA maynot be appropriate. �13C results for TCA, however, weresimilar between 1995 and 1999 PPG batches �−25�86 and−25�78‰�.

In 2003 Shouakar-Stash examined �13C values for TCEfrom five different manufacturers used in the van Warmer-dam et al. (1995) investigations. One conclusion of the sub-sequent study was that the samples had not degraded fromthe 1995 study. Contrary to the results of Beneteau et al.,(1999), Shouakar-Stash found that �13C values for TCE andTCA did have characteristic isotopic signatures associatedwith the respective manufacturer. The difference in resultsis probably due to the use of the Parr Oxygen Bomb method-ology in the earlier research, which resulted in low yieldsand loss of sample (Jendzejewski et al., 1997). A similarpattern was found for �37Cl values for different manufac-turers regardless of whether the compound was TCE orTCA. The consistent �37Cl signature for batches from dif-ferent years is consistent with the conclusion that each

manufacturer has a characteristic �37Cl value. An explana-tion for the variation in �37Cl values between manufacturersis due to isotopic fractionation occurring during the pro-cessing of the source brines used to produce chlorine gas.Other researchers indicated that a more negative �37Cl valueimplied a biodegraded chlorinated solvent (Sturchio et al.,1998, 2002; Reddy et al., 2000; Numata et al., 2002). Theseresults suggest that �13C values may be used to distinguishbetween TCA manufactures.

Shouakar-Stash identified differences in �2H attributableto different manufacturers although large errors are associ-ated with this analysis due to impurities in the sample. Oneaspect of the �2H analysis was its use to distinguish betweenmanufactured TCE and TCE resulting from the dechlorina-tion from PCE. The authors found that the hydrogen atomon TCE does not undergo an exchange reaction with thesurrounding water, thus any change in the hydrogen signa-ture should be associated with the degradation of TCE, viacarbon isotope fractionation. An analysis of TCE from differ-ent manufacturers exhibited a completely different �2H sig-nature than from TCE produced from PCE dechlorination.Manufactured and dechlorinated TCE (degraded from PCE)�2H values ranged from +466�9 to +681�9‰ and −351�9 to−320�0‰, respectively. An extension of this relationship is ifa linear relationship between �2H and �13C exists for differ-ent manufactures, then a means exists to identify the sourceof TCE in a series of samples collected along the axis ofa contaminant plume. Table 12.6.2 summarizes the resultsof the �13C values for different manufactured chlorinatedsolvents.

From a forensic perspective, releases of chlorinated sol-vents from different manufacturers may be difficult toestablish due to the precision of the analyses, effects ofdissolution, sorption, and volatilization, whose extent aredifficult to quantify, as well as significant differences in the�13C from the same supplier. Researchers have postulatedthat a TCE sample obtained in the field that may be affectedby any of these processes is impossible to isotopically dif-ferentiate between sources that differ by less than 1.0 or±0�5‰ (Slater, 2003). It is also difficult to reliably interpretsmall variations in isotopic composition to different sourcesas viewed from a confidence interval perspective. The stan-dard deviation of multiple, independent analyses of a samplecan range from 0.1 to 0.3‰ (represents a confidence intervalof 68% or one standard deviation). In order to delineate a95% confidence level, two standard deviations are required,which correspond to a range of 0.2–0.6‰.

Table 12.6.2 Comparison Between Recent �13C Value for TCE and TCA from Different Sources and Years (afterShouakar-Stash et al., 2003)

Compound

Shouakar-Stash et al., 2003 van Warmerdam et al., 1995 Beneteau et al., 1999

n

Mean�13CVPDB

(‰)STDEV

1� n

Mean�13CVPDB

(‰)STDEV

1� n

Mean�13CVPDB

(‰)STDEV

1�

TCE DOW 92 7 −31�57 0.01 2 −31�90 0.05 — —TCE DOW 95 4 −29�33 0.10 — — 3 −29�84 0.07TCE PPG 93 4 −27�37 0.09 2 −27�80 0.01 — —TCE PPG 95 4 −31�12 0.06 — — 3 −31�68 0.01TCE ICI 93 4 −31�01 0.09 3 −31�32 0.03 — —TCE StanChem 93 3 −29�19 0.14 — — —TCA ICI 93 5 −26�48 0.18 4 −26�64 0.09 — —TCA PPG 93 3 −26�17 0.12 3 −25�80 0.46 — —TCA PPG 95 5 −25�84 0.14 — — 4 −25�78 0.13TCA Vulcan 593 3 −28�54 0.17 3 −28�42 0.07 — —TCA StanChem 93 6 −27�39 0.10 — — — —


An issue in interpreting isotopic data for chlorinated sol-vent source differentiation is temporal source variation aswell as the assumption that the isotopic characteristics ofthe released chlorinated solvent(s) are known (Slater, 2003).In practice, it is rare that a historical sample of a chlorinatedsolvent is available to provide a baseline isotopic signature.If a release of TCE into the environment is from a singlerelease, then the source �13C is the �13C of the TCE thatentered the subsurface. If the TCE released into the sub-surface is the result of multiple releases over time, thenthe �13C value of the TCE is an isotopic mass balance ofthe total amount of TCE released. In a field investigation ofTCE released into an anaerobic, unconfined aquifer locatedin a glacial till, additional challenges in data interpretationincluded the inability to identify discrete sources unless nei-ther degradation nor isotopic fractionation was occurring,temporal variability, data density issues, and difficulties inquantifying the relationship between isotopic fractionationand degradation (Slater et al., 2000, 2001).

Kloppmann et al. (2005) used carbon specific isotopicanalysis analyses for PCE, TCE, and cis-1,2-DCE in additionto tritium analyses and major ion chemistry to character-ize a chlorinated solvent plume at the Plaine des Boucherssite, which is an old industrial area of Strasburg in usesince 1910 (Zwank et al., 2003; Kloppmann et al., 2005).The site is situated on highly permeable alluvial deposits ofthe Rhine Valley. �13C Values of PCE in monitoring wellswere found to be in a narrow range of −24�5 ± 0�48‰; thisinformation along with concentration data allowed the iden-tification of two PCE source areas. The TCE and cis-1,2-DCEanalysis exhibited enriched �13C with respect to PCE andwas comparable to measured ranges for unaltered industrialproducts (Beneteau et al., 1999; Jendrzewski et al., 2001).The observed TCE and cis-1,2-DCE �13C values were foundto range from −27�6 to 8.54‰ and −14�7 to 13.5‰, respec-tively, which are not representative of unaltered indus-trial values and therefore are evidence of biodegradationof TCE.

Issues of temporal variability are frequently encoun-tered in environmental forensic investigation and representtremendous challenges, especially where historical informa-tion regarding the release history and chemical usage isunknown. The ability to discriminate between �13C valuesresulting from multiple releases of product from differentmanufacturers with different �13C values that are co-mingledintroduces significant difficulty interpreting isotopic data forsource discrimination purposes.

Sherwood Lollar et al. (1999) proposed criteria when eval-uating isotopic data to distinguish between differences dueto degradation versus source differentiation. These criteriaare that the isotopic values must be sufficiently different soas to distinguish between different sources, that the isotopicdifferences are greater than the precision at which the iso-topic compositions of the compounds of interest are known,and that the isotopic behavior of the compound must be pre-dictable (i.e., the effect of environmental processes on theisotopic values of the chlorinated solvents must be known).A suggested approach to resolving some of these issuesis to perform laboratory studies in controlled conditions tosimulate the expected field environment as a baseline forproviding discrimination for many of the interpretative chal-lenges when evaluating isotopic data.

12.6.3 Ratio Analysis For Age-Dating a TCA SpillGauthier and Murphy (2003) describe a method of age-dating a TCA spill to groundwater based on the ratio ofdaughter degradation products to the parent compound.The method is based on the fact that the rate of hydrolysisof TCA to 1,1-DCE appears to depend only on groundwater

temperature and not on other factors including sorption.The variation of the hydrolysis molar rate constant k withgroundwater temperature is significant, occurring throughthe Arrhenius equation:

k = AeE

RT � (12.6.2)

whereA is the Arrhenius constant,E is the activation energy,T is the groundwater temperature in degrees Kelvin

(K), andR is the gas constant �8�3145×10−3 kJ/mol −K�.

Based on a review of uncertainties and biases in thelaboratory data, Gauthier and Murphy recommend valuesA = 8�7×1013 sec−1 and E = 122�8 kJ/mol−K. Groundwatertemperature can either be measured or, for shallow ground-water, can be estimated as equal to the annual average airtemperature. For deeper groundwater it may be necessaryto consider that except in geothermal regions there is abouta 1 �C increase in temperature for every 40 meters of depthbecause of the earth’s thermal gradient.

As shown in Figure 12.3.2, TCA also biodegrades anaer-obically to 1,1-DCA and then to chloroethane. Becauseanaerobic dechlorination is faster for the first step than thesecond, 1,1-DCA accumulates. Similarly, if conditions arenot too reducing, the conversion of TCA to 1,1-DCE is fasterthan the loss of 1,1-DCE to vinyl chloride. In that case theeffective3 molar rate constant for biodegradation, kb, canbe estimated from the hydrolysis rate constant, k, and themolar concentrations of 1,1-DCE and 1,1-DCA:

kb = k�1,1 DCA

�1,1 DCE� (12.6.3)

where square brackets indicate molar values.The kinetic equation is given by Gauthier and Murphy

(2003) as:

t = 1�k+kb�

ln(

1+ 1

(�1,1 DCE

�TCA

)+(

�1,1 DCA

�TCA

))sec�

(12.6.4)

where t is the length of time since hydrolysis began and isthe molar fraction of 1,1-DCE produced by TCA hydrolysisestimated by Gauthier and Murphy (2003) to be about 0.21.Equations 12.6.2 through 12.6.4 and parameter values aresufficient to calculate the time t. When time series con-centrations are available, plotting the logarithm in equa-tion (12.6.4) vs t gives a direct estimate of k+vkb, so use ofEquation (12.6.2) is unnecessary. An example, provided byWing (1997) is shown as Figure 12.6.1. For this example,the spill to a shallow aquifer occurred on August 15, 1984.

Gauthier and Murphy (2003) estimated the uncertaintyin t as about ±25% when laboratory data are used to esti-mate rate constants. For the data used by Wing (1997),Gauthier and Murphy estimate the uncertainty as ±25% forone groundwater monitoring well and ±36% for anotherwell.

An important observation is the interpretation of t. Onlynear the contaminant plume front is the rate of hydrolysisthe same as the time since TCA entered groundwater. Inparticular, when pure solvent is introduced to groundwateras a dense nonaqueous phase liquid (DNAPL) one expectshydrolysis to begin with dissolution of the outer surface of

3� The actual rate constant may vary in space and time. For exam-ple, biodegradation may only occur in an anaerobic region near thesource.


Figure 12.6.1 Wing (1997) Analysis for t (x-intercept) and k (slope).

the DNAPL, which is also when transport and biodegra-dation begins. As noted by Gauthier and Murphy (2003),laboratory experiments demonstrate that only the dissolvedTCA hydrolyzes. Thus, as successive DNAPL layers are dis-solved, hydrolysis begins at a later time. The net effect isthat the data curve in Figure 12.6.1 should decrease in slopeand become a horizontal line. This “bending over” in fact iswhat is observed at later times in the data used by Wing.When the logarithm is constant, the time calculated shouldcorrespond to the transport time from the DNAPL regionto the well in which the data were recorded.

12.6.3.1 Ratio Analysis for Age-Dating OtherChlorinated Solvents

The question naturally arises whether ratios, for example of1,2-DCE to TCE can be reliably used to age-date releases togroundwater. Because the degradation product in this caseis a result of biological activity, the situation is clearly morecomplex than the use of a simple exponential decay modelto develop a half-life degradation to estimate the date of thechlorinated solvent release. In addition to temperature, addi-tional variables, such as the type and population of microbespresent, the presence of nutrients or electron acceptors, thetime required for microbial acclimatization to a spill, per-cent organic carbon, mass and chemical composition of thedegradation products in the original mass released and soiltexture. The concentration of the chlorinated solvents (dis-solved or free phase) released into the subsurface is alsoimportant when considering this approach as the ability ofmicro-organisms to biodegrade chlorinated hydrocarbons at

high concentrations is considerably reduced. It may be pos-sible in some unique circumstances to bracket a release datefrom ratios but, in general, consistency with age estimatesdeveloped by other means is probably more accurate.

To demonstrate consistency, biodegradation rates canbe determined by comparing results of a transport andbiodegradation model, such as Biochlor, with field data.Through trial and error, effective biodegradation rates canbe estimated. The rates are “effective” because they repre-sent an integration over space and time. These biodegrada-tion rates can then be compared with literature values todemonstrate consistency. Of course, there is still an issue asto which literature values, field or laboratory, best representconditions at the site of interest.

12.6.4 Ratio Analysis for Source IdentificationAlthough using chlorinated ethane ratios for age-dating isusually problematic, these ratios can be used to identifyadditional sources. Figure 12.6.2 shows an example fromthe Tutu well field in St. Thomas in the Virgin Islands. Theplot shows the molar ratio of 1,2-DCE to the sum of TCEand PCE. In this case, PCE is believed to be the parentcompound. Molar ratios are used so that numerator anddenominator both correspond to number of molecules thatare or were formerly PCE, but that are different levels inthe anaerobic decay chain depicted in Figure 12.3.1.

In this example there is a sudden ratio reduction at about1500 feet downgradient where there is a dry-cleaning estab-lishment. In general, in order to reliably determine a sourcelocation the change in ratio should be statistically significant

Figure 12.6.2 [DCE]/{[TCE[+[PCE]} vs. Downgradient Distance at the Tutu Wellfield, St. Thomas, Virgin Islands.


compared with upgradient fluctuations. In Figure 12.6.2 thedownward trend in the ratio may be due to aerobic degrada-tion or preferential loss of some compounds in the bedrockmatrix. Because this method is phenomenological, it is notnecessary to know the underlying processes.

In the example shown in Figure 12.6.2 the change in ratiosoccurs at only one groundwater monitoring well. There is agroundwater divide near monitoring the dry-cleaner locationso that wells shown in Figure 12.6.2 as downgradient may infact not be relative to the drycleaner discharge location. Ingeneral, other reasons that only one or a few wells may beaffected include the release being very small or very recent.In other cases one would expect a number of downgradientwells to be affected.

12.6.5 Age-Dating a Chlorinated Solvent Release fromthe Position of the Plume Front

The age dating of a chlorinated solvent release via thelocation of the leading edge of the contaminant plume ingroundwater is frequently used for age-dating chlorinatedsolvent plumes. Chlorinated solvent plumes are less subjectto biodegradation than petroleum product constituents suchas benzene, toluene, ethylbenzene, and xylene (BTEX):often there is a source region where anaerobic degradationoccurs but then the plume enters an aerobic region wherethe more heavily chlorinated compounds such as PCE orTCE biodegrade little if at all. Thus the position of theplume front for the parent compound is more likely to bedetermined by travel-time considerations than biodegrada-tion processes.

This forensic technique is based on an analytical approachsuitable for a homogeneous and isotropic medium. Whenthe medium is inhomogeneous or anisotropic, numericalmodeling should lead to more accurate results than a sim-ple analytical formulation. However, numerical modelingwill not reduce the uncertainty in plume age resulting fromuncertainties in the basic parameter values.

In this example, assume that the length of the chlorinatedsolvent plume is L. Some portion of L� �L, is due to longi-tudinal dispersion. If the groundwater velocity is u and theretardation factor for the solvent in question is R, so thatthe solvent transport velocity is u/R, then the release date,t, can be estimated as:

t = �L −�L�R

u(12.6.5)

The longitudinal dispersion can be estimated as:

�L = 2

√Dt

R= 2

√Lut

R= 2

√L�L −�L�� (12.6.6)

where D is the coefficient of hydrodynamic dispersion, L isthe longitudinal dispersivity, and molecular dispersion hasbeen neglected. The factor of two is included because thisappears as part of the characteristic length scale in two-and three-dimensional analytical solutions (Domenico andSchwartz, 1997). The approximate solution for �L is:

�L � 2√

LL −2 � 2√

LL (12.6.7)

Thus combining Equations (12.6.5) and (12.6.6) gives:

t =(

R

u

)(L −2

√L)

� (12.6.8)

which shows that the importance of correcting for longitu-dinal dispersion decreases with the length of the plume.

The groundwater velocity and retardation factor can bewritten in terms of measurable quantities as:

u = KI

�e

(12.6.9)

and

R = 1+ bKd

�e� (12.6.10)

where K is the hydraulic conductivity, I is the hydraulicgradient over the plume extent, �e is the effective poros-ity or interconnected pore space of the aquifer, b is theaquifer material dry bulk density, and Kd is the adsorptiondistribution coefficient.

The adsorption distribution coefficient Kd is often writ-ten as:

Kd = Koc foc� (12.6.11)

where Koc is the distribution coefficient for partitioning ofthe solvent between water and organic carbon and foc is thefraction organic carbon in the soil.

With substitutions from Equations (12.6.9–12.6.11) theexpression for the elapsed time since a release to ground-water becomes:

t =(

�e

K i

)(1+ bKoc foc

�e

)(L −√

2L)

(12.6.12)

For determining the chlorinated solvent plume length, thereare generally two cases. First, the chlorinated solvent maynot be detected at a distant groundwater monitoring wellbut may be detected at the next closest well to the source.Second, the chlorinated solvent may be detected at all dis-tant wells but with a declining concentration at the mostdistant well. In both cases it is important to determine thatthe observed decline in concentration represents a contam-inant front and not just a decline due to lateral dispersionor biodegradation. It may be helpful to plot centerline con-centration versus distance downgradient to determine if thedecline in concentration is more rapid than would be causedby dispersion or degradation alone.

If degradation is occurring along the entire extent ofthe groundwater contaminant plume (rather than just in asource region) it may be helpful to use molar concentra-tions and to add the concentrations of daughter productsto the concentration of the original solvent. An example ofdegradation along the entire plume length is hydrolysis of1,1,1-TCA to 1,1-DCE.

Another parameter used in this methodology is hydraulicconductivity. Hydraulic conductivity (units of length overtime, LT−1) is related to permeability k�L2� as follows:

K = g

�k� (12.6.13)

where is the fluid density �ML−3�, g is the acceleration ofgravity �LT−2�, and � is the kinematic viscosity �ML−1T−1�.

Most groundwater texts provide ranges and typical valuesfor K and k for various rock and soil types (Freeze andCherry, 1979).

Often a few hydraulic conductivity measurements areavailable. According to Zheng and Bennett (1995), “ampleevidence has suggested that within a given hydrogeologi-cal unit, hydraulic conductivity often follows a logarithmicnormal (or lognormal) distribution. They cite Law (1944)and Bennion and Griffiths (1966) as a basis. This suggeststhat the geometric mean, rather than the arithmetic mean,is a better representation. Beacause the geometric mean isalways less than or equal to the arithmetic mean, using thegeometric mean rather than the arithmetic mean has theeffect of weighting low conductivity regions more heavily.The effect is similar to an electrical circuit with resistancesin parallel where the more resistive elements control thecurrent flow.

REFERENCES 273

Porosity can be measured in the laboratory, as the ratioof pore volume to bulk volume (Zheng and Bennett, 1995).However, �, appearing in the above equations, is the effec-tive or connected porosity, which will be smaller. Effectiveporosity will be much smaller if a high percentage of theflow occurs through a small fraction of the pore space.

Bulk density is the mass of dry soil divided by bulk vol-ume. In homogeneous media it is related to porosity andparticle density by b = p�1 − ��, where p is the particledensity. A typical particle density is that of silicon dioxide,2�65 g/cm3.

Gelhar et al. (1992) estimated dispersivities from field dataat a number of sites. Their data are classified by the degreeof reliability assigned by the authors. Without regard to reli-ability, the data indicate increasing with scale (distance).However, when only data of high reliability are considered,dispersivity appears to change very little with scale. In con-trast, the high reliability data are clustered at intermediatescales making a trend difficult to discern. Gelhar et al. alsonoted no significant difference between porous media andfractured rock. Gelhar et al. also analyzed a smaller numberof observations in which the ratio of longitudinal to trans-verse or vertical dispersivity could be determined.

Changing groundwater flow direction has some effect onapparent longitudinal dispersivity but it is much less thanthe effect on apparent transverse dispersivity (Goode andKonikow, 1990).

The ith parameter value is denoted as xi . When parame-ter values are uncorrelated, the uncertainties are normallydistributed,4 and the uncertainty in each, �xi , is small sothat �xi/xi � 1, then the uncertainty in elapsed time is (Man-del, 1964)

�t =(

n∑i=1

(�t

�xi

)2

V�xi�

) 12

� (12.6.14)

where V�xi� = ��xi�2 is the variance of the ith variable.

Equation (12.6.14) can also be written as:

�t

t=√√√√ n∑

i=1

(xi

� ln t

�xi

)2V�xi�

x2i

(12.6.15)

For normally distributed parameters√

V�xi �

xi= �i

xi, where �I

is the standard deviation of the ith variable. Even whenthe inequality �xi/xi � 1 is not satisfied, Equation 12.6.15provides a useful way of assessing which parameters aremost responsible for the fractional uncertainty in elapsedtime �t/t.

The values of V�xi �

xidepend on site-specific circumstances.

However, it is clear that in general, significant uncertaintyexists. Suppose, for example, that each of the three factorsin Equation 12.6.15 has a normally distributed uncertaintywith a value of ��xi �

xi= 0�3. Then the uncertainty in �t

tis about

0.52. This would correspond, for example, to a range of5–15 years with a central estimate of 10 years. In many casesthis degree of uncertainty will prevent detailed forensic con-clusions, such as “during whose ‘tenure’ did the releaseoccur” from being concluded. The most precise conclusionsthat can then be drawn are confirmatory, namely that withreasonable parameter values the plume extent either is, oris not, consistent with other information. Thus, it may be

4� Note that this is not the same thing as a normally distributedparameter. According to the Central Limit Theorem the uncertaintyin an unbiased estimate of the mean for most distributions is approx-imately normal.

possible to disprove other estimates that are independentlyarrived at.

In general, collecting additional hydraulic conductivityinformation, seasonal hydraulic gradient information, etc.may reduce uncertainties. Additional observations may alsoreduce uncertainty in specific circumstances as the follow-ing examples illustrate.

When retardation is small, the value of R is only slightlylarger than one. An example is the stabilizer 1,4-dioxanefound in 1,1,1-TCA. According to Mohr (2001) the estimatedKoc for 1,4-dioxane is 1.23. This means 1,4-dioxane is nearlyunretarded in groundwater transport. Basing the elapsedtime estimate on 1,4-dioxane rather than 1,1,1-TCA thereforeeliminates the uncertainties associated with the retardationfactor. Of course, this reduction in uncertainty could be par-tially or totally cancelled if the position of the 1,4-dioxaneplume front is less precisely determined than the 1,1,1-TCAplume front. In contrast, because it is the fractional uncer-

tainty

√V�L−

√L�

L−√L

that is of interest, the larger value of L for1,4-dioxane relative to 1,1,1-TCA may actually reduce thisquantity.

(Similarly, MTBE may be used to date gasoline plumeslargely avoiding the uncertainties associated with retarda-tion and biodegradation affecting the position of the plumefront. In the special case where an MTBE plume lags behinda BTEX plume this indicates that there was an earlier spillof non-MTBE petroleum.)

If the date one chemical entered groundwater t∗ can bereliably estimated, the position of the plume front L∗ for thatchemical may be used to estimate the retarded groundwa-ter velocity, u∗/R∗, for that chemical. The date that otherchemicals entered groundwater is then given by:

t = t∗u∗Ru R∗

(L −√

L)

(L∗ −√

L∗) � (12.6.16)

In writing Equation 12.6.16, we have not formally recog-nized the possibility that is a function of downgradientdistance L.

When a multi-component contaminant has been spilledthere may be multiple plume fronts. An example would bethe various polycyclic aromatic hydrocarbon (PAH) plumefronts from a creotote release to groundwater. Because thehigher ring-number PAHs are more heavily retarded thanthe lower ring-number ones, chromatographic separationoccurs. In this case the estimated plume age can be calcu-lated. The estimated value of L −√

L is plotted versus theestimated values of 1/R for each component. The resultingbest-fit straight line is an estimate of ut and the scatter aboutthe line provides an estimate of the uncertainty in ut.

Finally, we mention a special case observed by one of theauthors where a pumping well installed as part of a remedyleft a “signature” in the plume at the capture radius. Observ-ing the subsequent progress of this plume “back end” pro-vided a direct estimate of the retarded transport velocity,albeit neglecting the effects of longitudinal dispersion.

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