Environmental Forensics: Principles and Applications - Chapter 2 pptx

Examples of refined products include gasoline, aviation fuels, jet fuel, andthe newer formulations of diesel fuels Harvey, 1998.. Themiddle distillate fractions are separated into kerose

2.2 CHEMISTRY OF CRUDE OIL

There are over one million types of hydrocarbons in crude oil, ranging from lightgases to heavy residues No two crude oils are identical Crude oil is defined by Philip(1998) as…

…extremely complex mixtures of saturated and aromatic hydrocarbons, ranging from

C1 to C100 or higher, plus a wide variety of compounds containing nitrogen, sulfur, and oxygen In addition, there is also a fraction called the asphaltene fraction which is

basically insoluble in n-pentane and contains a very complex matrix of high molecular

weight polar compounds.

In most cases, 90 to 98% by weight of crude petroleum consists of hydrocarbons,while the remaining materials include sulfur, oxygen, nitrogen, and other organiccompounds Variations in crude oil composition occur due to the nature of the source

of the organic material, the geologic and thermal history, chemical changes that occurduring oil formation and migration, and chemical alteration due to biodegradation,oxidation, or selective dissolution Despite wide variations in the chemistry of crudeoil, the elemental compositions fall within a narrow range of elements, as shown onTable 2.1 (Neumann et al., 1981) Crude oils have normal paraffins (n-paraffins)

ranging from C1 to C40 Although higher carbon numbers exist in crude oils, mostcrude oils fall within the C5 to C30 range (Schmidt, 1998).

The predominant hydrocarbon classes that comprise crude oil are straight or chain alkanes, cycloalkanes, and aromatics Alkanes (paraffins) are saturated hydrocar-

branched-bons Linear or normal alkanes (n-alkanes) ranging from C1 to C40 have been

identified in crude oil and usually comprise 15 to 20% In general, the most abundantalkanes in crude oil are the low-molecular-weight normal alkanes (C5–10) Normal

alkanes (n-alkanes) are linear chains of carbons linked by single covalent bonds.

Isoalkanes are hydrocarbons containing branched carbon chains The highestconcentration of isoalkanes in crude oils is in the C6 to C8 range Crude oil cancontain 10 to 15% isoalkanes

Cycloalkanes are similar to alkanes except that cycloalkanes consist of rings ofcarbon atoms joined by single atomic bonds Cycloalkanes are abundant in crude oilsand can comprise up to 30 to 40% by weight The most abundant cycloalkanes (alsocalled naphthenes) are the single-ring cyclopentanes (C5H10) and cyclohexanes (C6H12).Steranes and triterpanes are complex cycloalkanes often used as markers to identifythe source and age of crude oil (Hughes and Holba, 1988; Seifert and Moldowan,1978; Stout et al., 1999)

Aromatic hydrocarbons consist of rings of six carbon atoms that are unsaturated(i.e., they do not contain the maximum number of bonded hydrogen atoms) Aromat-ics include the BTEX (benzene, toluene, ethylbenzene, and total xylenes) and poly-nuclear aromatic compounds (PNAs) Aromatic hydrocarbons contain carbon atomslinked with double bonds, the simplest being benzene (C6H6) Each hydrogen atom

on the aromatic ring can be replaced with an alkyl group (CH3) which results incompounds such as toluene with one alkyl group attached to the benzene ring.Benzene rings can be linked to other benzene rings to form compounds such asbiphenyls or terphenyls When two or more benzene rings are fused, polynucleararomatic hydrocarbons (also known as polycyclic aromatic hydrocarbons, or PAHs)are formed (see Section 4.10 in Chapter 4) Polycyclic aromatic hydrocarbons arecompounds that originate from crude oil and many pyrolysis processes Polycyclicaromatic hydrocarbons are of concern because of their genotoxic properties Naph-thalene (C10H8) is a lower molecular weight example and is generally considered to

be a polycyclic aromatic hydrocarbon, although it has only two aromatic rings Othernon-hydrocarbon components in crude oil include sulfur, oxygen, and nitrogen

TABLE 2.1 Elemental Composition of Crude Oil

Sulfur is typically the most abundant element and may be present in several forms,including elemental sulfur, hydrogen sulfide, mercaptanes, and thiophenes (i.e.,hydrogen molecules with bonded sulfur atoms) The sulfur content in most crude oilsvaries from about 0.1–3% for some of the heavier oils to 5–6% for bitumen Sulfurdoes not decompose during the distillation process The majority of sulfur is, there-fore, present predominately in the higher molecular weight fractions and becomesconcentrated in the higher weight refined products The analysis of the sulfur content

of crude and refined products, such as diesel, can be used to provide evidence todistinguish between multiple sources The sulfur content of a petroleum hydrocarbon

is determined using standards such as American Society for Testing Materials(ASTM) D-124, D-1552, and D-4294

Oxygen reacts with hydrocarbons to form compounds such as phenols, cresols,and xylenols Nitrogen can bond with hydrocarbon molecules in crude oil to formsmall concentrations of pyrrole, pyridine, and quinoline Metals are present in crudeoils, although usually in small amounts Metals can occur as inorganic salts, metallicsoaps, and organometallic compounds In some instances, sodium arsenite andarsenic trioxide are added to oil pumping wells to inhibit corrosion (Rohrbach et al.,1953; Wellman et al., 1999) The presence of arsenic in crude oil may, therefore,provide a means for identifying the origin of the crude oil

2.3 CHEMISTRY OF REFINED PRODUCTS

The chemistry of a refined petroleum product is the result of the composition of thecrude oil and the refining process The term “refined products” refers to thosepetroleum hydrocarbons that are blended and to which additive packages are in-cluded Examples of refined products include gasoline, aviation fuels, jet fuel, andthe newer formulations of diesel fuels (Harvey, 1998) Major refinery processes thataffect product chemistry are (Speight, 1991):

• Separation of the crude oil into various fractions

• Conversion of marketable portions of the crude oil

• Finishing of the various product streams

Separation and removal of the various portions of crude oil have historically beenaccomplished via distillation The three products created via distillation are naphtha,middle distillates, and residual hydrocarbons Naphtha, with a boiling range of 90 to

190∞C, includes gasoline, which is further processed for octane improvement Themiddle distillate fractions are separated into kerosene (light-end) and diesel range(heavy-end) products The light-end middle distillates (boiling ranges from 150 to

260∞C) include kerosene, mineral spirits, Stoddard solvent, jet fuels, and diesel No 1.Stoddard solvent was used extensively in the first half of this century for degreasingbut was replaced by chlorinated solvents such as trichloroethylene due to the poten-tial fire hazards associated with Stoddard solvent (Stewart et al., 1991) Examples ofheavy-end products are Bunker fuels, heavy fuel oils, and asphalt Examples of chro-matograms for mineral oil, Stoddard solvent, and kerosene are shown in Figure 2.1

FIGURE 2.1 Chromatograms of mineral oil, Stoddard solvent, and kerosene (From Bruya,

J., Chromatograms, Friedman and Bruya, Seattle, WA, 1999 With permission.)

Heavy-end middle distillates with boiling ranges of 190 to 400∞C are processed

to produce diesel fuel No 2 and heating oils (Kaplan et al., 1995) Table 2.2summarizes key distilled products, their distillation temperature range, and carbonrange (Galperin, 1997; Schmidt, 1998) While the distillation temperature and Ameri-can Petroleum Institute (API) gravity of hydrocarbons provide useful information inthe refining process, they can provide corroborative evidence in distinguishingamong multiple sources of fuel releases API gravity is defined in Equation 2.1 as:

API gravity = 141.5/P – 131.3 (Eq 2.1)where P is the specific gravity of the crude oil or refined product at 60∞F Evidenceused to distinguish among sources of diesel, gasoline + diesel + jet fuel, and gasoline

at a refinery is shown in Figure 2.2 as a function of API gravity The API gravity ofeach of the various fuels stored at the refinery were known, thereby providing abaseline for comparison

The use of the distillation temperature of a fuel to distinguish among multiplesources (degraded gasoline and a gasoline + diesel + jet fuel mixture) is shown onFigure 2.3 For the free product samples collected from the groundwater table shown

in Figure 2.2, the initial boiling point (IBP) and final boiling point (FBP) of the fuelswere known, thereby allowing correlation of the IBP and FBP of the samples tospecific locations on the refinery

The evolution of crude oil refining over time has resulted in different productsand blends of refined product The unit process and the waste streams from theseprocessing changes are helpful in age-dating a product and/or bracketing a timeframe when certain refinery processes and their associated waste products wereproduced Table 2.3 summarizes some of the key historical changes in petroleumrefining (Gibbs, 1990; Harvey, 1998)

Kerosene and jet fuels 150–250 C11–C13

Diesel and fuel oils 160–400 C13–C17

exhibits an asymmetric distribution pattern from the CH1 (methylcyclohexane) toCH7 (a heptylcyclohexane) range, with the CH2 peak being the most abundant andthe peaks CH2 to CH7 decreasing rapidly in intensity (Galperin, 1997) Gasoline has

a boiling-point distribution from about 120 to 400∞F As a result of the preferentialpartitioning of low-boiling-temperature compounds found in gasoline, the concentra-tion of the BTEX components can be as high as 1 to 4% for benzene and 3 to 20%for toluene

Gasoline blending has changed, in part, to create fuels with different octane ratings.Examples of gasoline grades are summarized in Table 2.4 (Harvey, 1998) Gasolineblends often reflect the level of refining A premium-grade gasoline, for example, is a

FIGURE 2.3 Use of initial (IBP) and final boiling point (FBP) temperatures to identify fuel types.

FIGURE 2.2 Use of API gravity to distinguish between fuels.

more tightly regulated blend than a mid-grade or regular gasoline Chromatograms

of gasoline grades and blends are shown in Figure 2.4 (Zemo et al., 1993).Changes in the octane ratings of different gasoline grades include a 65 to 75octane rating in 1910, an average octane rating of 82 in 1946, and an average octanerating of 96 in 1968 (Gibbs, 1990) The significance of these different gasoline gradesand octane ratings over time is that it is unlikely that forensic testing can identify agasoline grade once it has entered the subsurface Compounds used to provide higheroctane ratings, however, can be identified on a chromatogram Examples include iso-octane, toluene, ethylbenzene, xylenes, and trimethylbenzene For example, a pre-mium-grade, 1994 gasoline tends to have a high percentage of iso-octane andaromatics The greater the combined percentage of iso-octane and aromatic com-pounds, such as toluene, the higher the octane and fuel quality and, therefore, themore likely it is that the product was refined and blended

TABLE 2.3

Chronology of Key Changes in Petroleum Refining in the U.S.

1910 Straight run (distilled) products produced; 65–75 octane rating

1913 Dubbs thermal cracking process introduced

1920 Coking introduced

1923 Lead introduced in gasoline to minimize backfiring

1926 Lead anti-knock additive introduced

1928 Lead scavengers ethylene dibromide and ethylene dichloride introduced

1929 Regular and premium gasoline sold

1936 Fluid catalytic cracking introduced

Super premium leaded Premium or supreme unleaded

Premium or supreme leaded Mid-grade unleaded

“Super regular” leaded Regular unleaded

Regular leaded

Economy leaded

Regular low lead

Refined gasoline contains olefins (alkenes and alkynes), while crude oils andvirgin naphthas do not As a result, olefins are useful for distinguishing betweenrefined and crude oils Olefins are products of the catalytic cracking process Olefinsare identified on chromatograms as a cluster of small peaks to the right of the C6 peak(Schmidt, 1998) Alkynes (acetylenes) are also not normally found in crude oil.Another indicator used to distinguish between refined and unrefined products is the

FIGURE 2.4 Gasoline chromatograms (From Zemo, D and T Graf, in Proc of the 1993 Petroleum Hydrocarbons and Organic Chemicals in Ground Water: Prevention, Detection, and Restoration, November 10–12, Houston, TX, Ground Water Management Book 17,

National Ground Water Association, Dublin, OH, 1993, pp 39–54 With permission.)

presence of methylcyclohexane (MCH) Methylcyclohexane is abundant in fined gasoline range hydrocarbons and naphthas In general, the greater the concen-tration of methylcyclohexane, the less the product has been refined, as well as thelower the octane rating of the gasoline.

unre-2.3.2 D IESEL

Diesel consists of hydrocarbons in the C11 to C18–27 range Depending on the grade ofdiesel, it contains a high concentration of cycloalkanes and smaller amounts ofaromatic compounds (i.e., BTEX) Diesel tends to have greater concentrations ofpolycyclic aromatic hydrocarbons than does gasoline The changing formulation ofdiesel provides opportunities for dating a release Prior to 1975, diesel was primarilystraight chained, while post-1975 diesel was thermally cracked This distinction can

be determined analytically with only several milliliters of product, thereby providing

a bracket of time when the product was available Chromatograms of a diesel fuel No

1 and No 2 are shown in Figure 2.5

FIGURE 2.5 Chromatograms for diesel fuels No 1 and No 2 (From Bruya, J., grams, Friedman and Bruya, Seattle, WA, 1999 With permission.)

Chromato-Diesel is available in various grades Chromato-Diesel, kerosene, and the lighter distillatescontain various amounts of the BTEX aromatics up to 1500 parts per million (ppm)(Dunlap and Beckmann, 1988) The composition and characteristics of diesels andmiddle distillate products are described in Table 2.5 (Havlicek, 1986; Kaplan et al.,1995; Kaplan and Galperin, 1996) A petroleum forensic laboratory to needed todistinguish between fuels that are similar Figure 2.6 illustrates chromatograms forjet fuel No 4 (JP-4) and jet fuel No 5 (JP-5) to show the chromatographic similarity

of these two fuels (Bruya, 1999)

TABLE 2.5

Products, Synonyms, and Characteristics of Diesel and Jet Fuels

Product and

Diesel No 1 Similar in composition to a blend of kerosene and diesel No 2 Diesel No 1 is

manufactured in cold climates and is also sold in warm climates when a refinery desires to blend its kerosene with more expensive diesel No 2 Diesel No 1 exhibits

an alkylcyclohexane pattern on a mass chromatograph in the range from CH 1 to

CH14, with a maximum at CH5 Diesel No 2 Automotive diesel Straight-run or catalytically cracked petroleum distillate with a

typical carbon range of C 8–9 to C 24–27 and a boiling range of approximately 163 to

382 ∞C Includes straight-run kerosene, middle distillate, hydro-desulfurized middle

distillate, and light catalytically and thermally cracked distillates Formulated for use in atomizing-type burners BTEX components can be present in small amounts Using gas chromatography/mass spectrometry (GC/MS), diesel No 2 shows a range of alkylcyclohexanes from CH1 to CH14, maximizing around the CH9 and

CH 10 peaks Characterized by a smooth n-alkane distribution pattern.

Diesel No 4 Railroad diesel A straight-run or cracked petroleum distillate with a typical carbon

range of C11 to C30 Used without preheating in commercial or industrial burners that can accommodate a higher viscosity diesel such as diesel No 4.

Diesel No 5 A fuel comprised primarily of straight-chained hydrocarbons Diesel No 5 is a

residual fuel that often requires preheating for handling.

Bunker C (heating A residual fuel used in commercial and industrial heating Bunker C requires

preheating for storage and for burning Sulfur is often found in higher tions than in other diesels, unless they are deliberately extracted Bunker C is the primary fuel for steam-powered ships and for onshore power-generation plants and

concentra-is primarily a mixture of diesel No 1 and No 2 and residual oil Bunker C concentra-is a distillation residue of crude oil and contains biomarkers such as terpanes and steranes Bunker C has a hydrocarbon range from C9 to about C36 and a boiling point range of about 340 to 1050 ∞F.

Kerosene A straight-run distillate with hydrocarbons in the C9/10 to C16 range A light-end

middle distillate used in vaporizing-type burners where the fuel is ignited by contact with a heated surface or radiation Consists primarily of paraffins with smaller amounts of naphthalene and aromatic hydrocarbons The carbon distribution peaks around C 12 to C 13 It is similar in composition to JP-5 and JP-6 jet fuels oil or diesel No 6)

(No 1 fuel oil)

2.4 CHEMICAL REACTIONS

IN THE VADOSE ZONE

The physical transport of crude oil and refined products through the subsurface is afunction of product chemistry, the hydraulic conductivity (K) of the soil or rock, andthe presence of a driving mechanism such as rainfall, ponded water, or leakage from

an underground storage tank An understanding of the relationship between a taminant and the media through which it is transported is used to estimate the relative

con-Stoddard solvent Used as a drycleaning solvent and paint thinner and in printing inks, certain

adhesives, and some photocopy toners It consists primarily of nonanes with smaller amounts of alkylbenzenes Synonyms include mineral spirits, light petroleum naph- tha, drycleaning safety solvent, petroleum solvent, varnoline, and spotting naphtha Registered trade names include Texsolve S ® and Varsol 1 ® (ATSDR, 1995) The boiling range is between 220 and 300 ∞F Stoddard solvent exhibits an alkyl-

cyclohexane pattern upon GC/MS in the CH2 to CH9 range, with the distribution maximizing at CH 5

Petroleum naphtha Naphtha exhibits an alkylcyclohexane pattern in the CH1 to CH6 range that

maxi-mizes at CH3.

Jet fuels:

JP-1 Military-grade distillate with a flash point of 95 ∞F.

JP-4 (Jet B) Military-grade distillate with a flash point of –10 ∞F and a boiling range of 48 to

270 ∞C Contains about 65% gasoline and 35% light distillates Most of the

volatile gasoline hydrocarbons are absent, and the iso-octane content is generally below 1% On a chromatogram, it looks like a light-kerosene and/or gasoline blend with a considerable amount of aromatic compounds Using GC/MS, it demonstrates a distribution pattern in the CH1 to CH9 range.

JP-5 (Jet A1) A U.S Navy distillate with a flash point of 95 ∞F and a boiling range of 150

to 290 ∞C JP-5 has a low freezing point and high flash point for use by

carrier-based aircraft for long-range flights JP-5 has an alkane distribution pattern in the kerosene range with a maximum around C 11 Using GC/MS, a distribution pattern in the kerosene range (CH1 to CH9) is discernible, with a noticeable difference in the maximum peak of distribution with CH5 for JP-5 and CH4 for Jet A.

JP-6 (Jet A) Military-grade distillate with a flash point of 100 ∞F Preferred for short- and

medium-range aircraft flights.

JP-8 A military aircraft fuel with a distribution pattern around C 10 or C 11 On a GC/MS,

an asymmetric distribution pattern is observable in the CH1 to CH14 range.

mobility and distribution of the contaminant Contaminant properties impacting themobility of a chemical through the unsaturated zone and saturated zones include itsHenry’s Law constant, vapor pressure, density, solubility, and viscosity If thepetroleum hydrocarbon has a significant volatile component, vapor transport of thecompound can be important.

The chemical and physical interaction of petroleum hydrocarbons in the face is important in understanding the mobility of the compound Commonlyencountered interactions include sorption, oxidation/reduction processes, chemicalprecipitation, ion exchange, hydrolysis, biological mediated reactions, and cosolva-tion

subsur-FIGURE 2.6 Chromatograms of JP-4 and JP-5 (From Bruya, J., Chromatograms, Friedman

and Bruya, Seattle, WA, 1999 With permission.)

For the lower alkanes (methane through hexane), the dimensionless Henry’s Lawconstant ranges from about 30 to 70 which means that, in equilibrium, 30 to 70molecules of these alkanes are present in the soil vapor for every molecule thatdissolves into the groundwater For the BTEX constituents, the Henry’s Law constant

is about 0.25, which means that one molecule of BTEX exists in the soil vapor for everyfour that dissolve into the water As a result, a soil vapor survey is about 200 times morelikely to detect the lower alkanes than BTEX compounds (Hartman, 1998)

2.4.2 L IQUID D ENSITY

The relative density (also called specific gravity) of a substance is defined as the ratio

of the density of the substance to the density of distilled water (a mass-to-volumeratio) The density of distilled water at a standard temperature and pressure is 1.0 g/

mL Specific density is a unitless measurement but is dependent on the temperature

of the substance at the time of measurement Light non-aqueous phase liquids(LNAPLs) have densities less than 1.0, while dense non-aqueous phase liquids(DNAPLs) have densities greater than 1.0 Most crude, residual, and used oils areLNAPLs with densities from about 0.6 to 1.0 g/mL

A contaminant’s density is important, especially when the contaminant enters thecapillary fringe (that partially saturated area above the groundwater table) Liquids withdensities greater than 1.0 (e.g., coal tar) have a greater probability of “sinking” intogroundwater than do liquids with densities less than 1.0 (gasoline, diesel, Stoddardsolvents, mineral oils), which tend to “float” on the water table BTEX (benzene,toluene, ethylbenzene, and xylene) compounds are lighter than water, while polycyclicaromatic hydrocarbons (PAHs) such as anthracene, chrysene, fluorene, naphthalene,

phenanthrene, and pyrene are heavier than water The term PAH is synonymous with

polynuclear aromatic hydrocarbons (PNAs) These chemicals are often classified ascarcinogenic — benzo(a)pyrene, benzo(a)anthracene, benzo(b)fluoranthene,benzo(k)fluoranthene, benzo(g,h,i)perylene, chrysene, dibenzo(a,h)anthracene, andindeno(1,2,3-c,d)pyrene — and noncarcinogenic — naphthalene, acenaphthylene,acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, and pyrene.Hydrocarbons with a high specific gravity are transported vertically in theunsaturated zone due to gravity and capillary forces If the volume of high-specific-gravity hydrocarbons released is large, the hydrocarbons will be vertically transported

through the soil and groundwater due to density This phenomenon is known asdensity flow Upon entering the groundwater, these hydrocarbons migrate as afunction of specific gravity and less by advection (the mass transport of groundwa-ter) Table 2.6 lists the densities of several light and dense hydrocarbons (API, 1989,

1994, 1995; Dragun, 1988)

2.4.3 S OLUBILITY

The solubility of a compound is the saturated concentration of the compound in water

at a known temperature and pressure The more soluble the compound, the greaterthe fraction that dissolves into the soil pore water or groundwater BTEX compounds

Bunker C (No 6 fuel oil) 0.969 0.974

Lube oil (crankcase oil, new) 0.878 —

Lube oil (crankcase oil, used) 0.885 —

are so frequently encountered in groundwater in part due to their high solubility.Benzene, for example, was detected 23% of the time at 888 Superfund sites based on

a total of 466 chemicals tested for, as of October 1986 Toluene and xylene were detected27% and 13% of the time, respectively Toluene was the second most frequentlyencountered contaminant, second only to trichloroethylene (TCE; 1100 mg/L at

20∞C) (Siegrist, 1993) Table 2.7 lists those highly soluble compounds in an API

PS-6 unleaded gasoline along with their estimated percent by weight (API, 1985, 1994).Compounds with high water solubilities tend to desorb from soils, are less likely

to volatilize from water, and are susceptible to biodegradation Compounds with lowsolubilities tend to sorb onto soils and volatilize more readily from water and aremore likely to enter the groundwater The water solubility of a compound varies with

pH, the presence of inorganic salts, and the presence of organic carbon Solubilities

of pure phase compounds in water at three temperatures are summarized in Table 2.8(Havlicek, 1986; Polak and Lu, 1973; Rossi and Thomas, 1981)

BTEX solubility in water is dependent on the nature of the multi-componentmixture, such as gasoline, diesel, or crude oil The solubility of a constituent within

a multi-component mixture may be orders of magnitude lower than the aqueoussolubility of the pure chemical constituent in water (Odencrantz et al., 1992) Theweight percent and mole fraction of the BTEX components as functions of themixture are also important Table 2.9 presents the calculated effective solubility ofBTEX compounds in gasoline, diesel, and a California crude oil (API, 1985; Metcalfand Eddy, 1993)

2.4.4 V ISCOSITY

Viscosity is the property of a substance to offer internal resistance to flow An idealfluid is one that is devoid of viscosity A similar but different term is “kinematic

TABLE 2.7

Highly Soluble Components of Gasoline

viscosity” which is the viscosity of the substance divided by its density The viscosity

of a liquid usually increases with decreasing temperature, though in some complexmixtures there is a discontinuity in the temperature/viscosity relationship Thesediscontinuities occur where there is a large change in viscosity over a very narrowtemperature range The simplest and most widely used determination of viscosity isAmerican Society for Testing Materials (ASTM) Standard Method D-445 as de-scribed in Equation 2.2

Effective Solubility of BTEX Components in Gasoline,

Diesel, and Crude Oil

Solubility of BTEX Compounds and MTBE

Solubility at 0∞C Solubility at 20∞C Solubility at 25∞C

While diesel and gasoline viscosities are similar, crude oil has a wide range ofviscosities For example, the viscosity of a Prudhoe Bay crude oil in Alaska is 73.5Saybolt units, while a Kern River crude oil from Bakersfield, CA, is greater than

6000 Saybolt units The viscosity of refined petroleum products varies from about0.25 to more than 50,000 cPa at 15∞C An approximate correlation between specificgravity and viscosity for refined products is described in Equation 2.3

hro = 8.28rro(9.5) (Eq 2.3)where

h ro = ratio of the kinematic viscosity to water.

r ro = specific gravity of the refined produced.

As a basis of comparison, the kinematic viscosity for water at 20∞C is 0.01 cm2/sec,while benzene has a kinematic viscosity of 0.00721 cm2/sec Pure benzene flowsabout 40% faster than water through identical porous media

The kinematic viscosity is called dynamic or intrinsic viscosity Infiltrationvelocities are often approximated as a proportionality that is inverse to the kinematicviscosity For example, a crude oil can migrate 3 to 35 times slower through soil thanwater (Dragun, 1988)

The kinematic viscosity of a hydrocarbon is affected by temperature For crudeoil, this effect can be several orders of magnitude Table 2.10 summarizes thekinematic viscosity of selected heavy fuel oils for two temperatures (API, 1989,1994; Dragun, 1988) A decrease in viscosity increases the flow rate of a hydrocarbonthrough a porous media During the natural weathering of petroleum products,viscosity tends to increase sharply

TABLE 2.10

Kinematic Viscosity of Refined Products

No 5 light fuel oil 130–400 5.5–8.0

No 5 heavy fuel oil 500–1200 9.0–13

a One centistoke (cSt) = 1 ¥ 10 6 m 2 /sec.

b Measured at 15 ∞C.

2.4.5 V APOR P RESSURE AND V APOR D ENSITY

Volatilization is the phase change of a compound from a liquid or solid to a gaseousphase; it is associated with the vapor pressure of the compound In general, com-pounds with vapor pressures exceeding 0.5 to 1 millimeter of mercury (mmHg) canexist in appreciable concentrations as vapor near a free phase product Hydrocarbonsthat volatilize quickly include butane, pentane, hexane, heptane, and octane Thearomatic BTEX and methyl ethyl benzenes and trimethylbenzenes also volatilizequickly but at a rate less than the butanes, pentanes, hexanes, heptanes, and octanes.The loss of BTEX compounds in a sequential order relative to vapor pressure is oftenobserved in the analytical data at hydrocarbon-impacted sites

Vapor density is the density of a compound relative to air (e.g., 29 g/mol) Mostpetroleum hydrocarbons have densities 3 to 4 times greater than air The vapordensity is estimated by dividing the molecular weight of a compound by the molecu-lar weight of air The molecular weight of benzene is 78 g/mol; therefore, dividing

78 by 29 yields a vapor density for benzene of 2.5 The vapor density for gasoline

is about 3.3 The significance of vapor density is that petroleum hydrocarbon vaporscan migrate through porous soils in the unsaturated zone as a function of vapordensity (Hartman, 1998)

2.4.6 S ORPTION

Sorption is the uptake of a vapor or liquid into another material without reference to

a specific mechanism (Chiou, 1989) Sorption encompasses the processes of tion, absorption, ion exchange, ion exclusion, retardation, chemisorption, and dialy-sis Of these processes, absorption (the penetration of substances into the bulk of asolid or liquid) and adsorption (the surface retention of a solid, liquid, or gasmolecules by a solid or liquid) are the most important This phenomenon results in

adsorp-a contadsorp-aminadsorp-ant’s distribution between the solid adsorp-and liquid phadsorp-ase adsorp-and reladsorp-ative retadsorp-ardadsorp-a-tion of the chemical The higher the fraction of the contaminant that is sorbed, the less

retarda-is available for transport

The sorption capacity of a compound is described by the term “sorption cient” The sorption coefficient is the ratio of an adsorbed chemical per unit weight

coeffi-of organic carbon to the concentration coeffi-of the contaminant It implicitly assumes areversible process — that is, sorption and desorption occur at the same rate

2.4.7 R ETARDATION

Retardation is the lowering of the average velocity of a contaminant mass relative

to the average (advective) groundwater velocity Compounds that sorb strongly toorganic carbon material in soils characteristically have low solubilities; compoundswith low tendencies to adsorb onto organic particles have high solubilities Theaffinity of a compound to be sorbed by organic or mineral matter is called theretardation factor (R) The retardation factor or coefficient is the ratio of the

concentration of a compound on a solid to the concentration of that compound insolution.

Laboratory experiments indicate that values for retardation vary widely, ing on the type of soil and contaminant Given the uncertainties associated withcalculating R, compounds with values less than 2 can be considered to be moving at

depend-a rdepend-ate simildepend-ar to groundwdepend-ater Distribution coefficient vdepend-alues cdepend-an be obtdepend-ained fromthe literature, calculated from the measured organic carbon content in soil, ormeasured from laboratory batch sorption or column transport studies The mostcommon technique is to measure the organic content of the soil and obtain the soil/organic carbon partition coefficient (Koc) of the chemical from published tables.Retardation directly impacts the rate at which a hydrocarbon or components of

a hydrocarbon will move in the subsurface For example, non-BTEX compounds thatare relatively mobile in water (based on their retardation coefficients) include 1,2,4-

trimethylbenzene, naphthalene, 2-methylnaphthalene, cyclohexane, n-hexane,

2,3-dimethylbutane, and 2,2-dimethylpentane Compounds that are relatively less mobile

in water values include benzo(a)anthracene, benzo(a)pyrene, 5-methylchrysene, methylphenanthrene, and dibenzothiophene

1-2.4.8 B IODEGRADATION

Biodegradation is the biological transformation of complex substances into simplersubstances by bacteria, fungi, and yeasts Hydrocarbon biodegradation is accom-plished primarily by bacteria Over 200 soil microbial species have been identified

that can assimilate hydrocarbon substrates Some of these microbes include nas, Flavobacterium, Micrococcus, Mycobacterium, Nocardia, and Acinetobacter (Bowlen and Kosson, 1995; Manahan, 1984).

Pseudomo-For most biodegradation pathways, the final degradation products are carbondioxide and water, a process called mineralization It is possible that the finalmineralization products are not achieved and that the degradation results in relativelystable aromatic hydrocarbons While a multitude of degradation reactions can occur,common transformations occur stepwise from end carbons, producing alcohols,aldehydes, and fatty acids in sequence

The rate and ability of microbes to degrade hydrocarbons is dependent on theability of the subsurface environment to support a healthy community of microbes.Conditions that influence the rates of hydrocarbon degradation include soil tempera-ture, soil porosity, soil moisture content, the oxygen content of the particle spaces,the nutrient content, and fuel type (Kaplan et al., 1995)

The concentrations of nutrients and oxygen required to sustain viable microbialcommunities are highly variable In general, a 1:20 ratio of available inorganicnitrogen to the petroleum hydrocarbon and a 1:100 ratio of available phosphate topetroleum hydrocarbons are necessary to support biological degradation of petro-leum hydrocarbons

Some generalizations concerning biodegradation are that biodegradation may notoccur if the concentration of the compound is very low and that most organic

compounds will degrade to some extent Current research suggests that benzene isnot degraded under denitrifying conditions and that toluene, xylene, and ethylbenzenedegradation are slow (Kao and Borden, 1997) At the Eglin Air Force Base site, the

preferential removal of toluene and ortho-xylene from the BTEX components closest

to the spill was observed Ethylbenzene and meta- and para-xylene degradation creased after the toluene and ortho-xylene were depleted (Wilson et al., 1994) It was

in-also calculated that for the BTEX components within the groundwater approximately1.0 mg of methane was produced for each 1.3 mg of BTEX destroyed In general, ifthe concentration of petroleum hydrocarbons or heavy metal concentrations are inexcess of 25,000 or 2500 ppm, respectively, then the environment is consideredinhibitory or toxic to aerobic bacteria Biodegradation commences as soon as thepetroleum hydrocarbon is released into the subsurface, with the lower molecularweight alkanes degraded first, followed by the higher molecular weight compounds.The temperature of the surrounding soil or groundwater also impacts degradationrates Some of the fastest bioattenuation rates of BTEX compounds observed by theEnvironmental Protection Agency’s Robert S Kerr Environmental Research Labo-ratory in Ada, OK, have been in cases where groundwater temperatures are high (24

to 28∞C)

Biodegradation rates are influenced by the molecular structure of the bon Straight-chained saturated hydrocarbons are degraded more readily than aro-matics (BTEX compounds), which are subsequently degraded more readily thanalicyclics and highly branched aliphatic hydrocarbons As a result of these hydrocar-bon degradation sequences, alicyclics and highly branched aliphatics accumulate inthe soil, while the more biodegradable of the compounds in the original product arenot present In general, the weathering of gasoline, diesel, and Bunker C fuelproceeds in the following temporal sequence (Galperin, 1997):

hydrocar-1 Abundant normal alkanes

2 Light-end normal alkanes

3 Middle-range normal alkanes, olefins, benzene, and toluene

4 More than 90% removal of the alkanes

5 Alkylcyclohexane and alkybenzenes

6 Isorepnoids and C0-naphthalene reduction

7 C 1 -naphthalenes, benziothiophene, alkylbenzothiophenes, and C 2 -naphthalenes

8 Phenanthrenes, dibenzothiophenes, and other polynuclear aromatic hydrocarbons

9 Tricyclic terpane enrichment, selective removal of regular steranes, reduction of

C31–C35 homohopanes

10 Increased abundance of tricyclic terpanes, diasteranes, and aromatic steranes

The biodegradation of specific petroleum fractions in a fuel has been proposed

as a means to age-date a hydrocarbon (Kaplan et al., 1995; Morrison, 1997; Raymond

et al., 1976) The basis of this approach is reliance on the biodegradation half-life ofhydrocarbon compounds in the soil or groundwater The estimated half-life is thetime required for one half of the compound to biodegrade The rate of biodegradation

is usually expressed in units of g/m2 day–1, g/m3 year–1, mg/day per bacterial cell,percent of oil removed after a known number of days or weeks, or g/m3 day–1 A