Chemistry of Petroleum

This appendix provides information about the composition and chemistry of PHC fuels and their vapors. This information may be used to identify certain petroleum chemicals that pose the greatest potential risks through PVI.

Aliphatic and Aromatic Compounds

The chemicals that make up PHCs can be divided into two general groups: aliphatic and aromatic compounds. Aliphatic compounds are composed of straight-chained, branched, or cyclic compounds and can be saturated (alkanes) or unsaturated (alkenes, alkynes, and others), whereas aromatic compounds have one or more conjugated, benzene or heterocyclic rings within their structures. “Conjugated” refers to the presence of delocalized (shared) electrons within the chemical structure, such as benzene. Some examples of aliphatic and aromatic compounds are shown in Figure C-1.

In general, aromatic and large aliphatic compounds (C9 and above) are more toxic than smaller aliphatic compounds (under C9); however, smaller aliphatic compounds are more volatile and are, therefore, generally the primary component in vapors if present in the petroleum source. Aromatic compounds are more readily biodegraded than aliphatic compounds; however, a few volatile aromatic compounds, such as benzene and ethylbenzene, and some semivolatile aromatic compounds, such as naphthalene, pose a carcinogenic risk whereas the aliphatic compounds are generally assumed to pose a noncarcinogenic risk. Although many aliphatic and some aromatic compounds may not be carcinogens, they can collectively pose a noncancer hazard if present at high enough concentrations and should be considered during PVI investigations.

Figure C-1. Examples of aliphatic and aromatic compounds.

Fuels as well as vapors are characterized in terms of individual compounds and carbon range groups of aliphatic and aromatic compounds. Physicochemical parameter values for aliphatic and aromatic carbon ranges as well as BTEX and naphthalene are presented in Table C-1.

Table C-1. Default physicochemical constants for BTEXN and TPH carbon ranges (adapted from Brewer et al. 2013)

Chemical/carbon range1

Molecular weight

Vapor pressure (atms)

Solubility in water (mg/L)

Henry’s constant (unitless)

Partition coeff, kc (cm3/g)

Diffusion coefficient (cm2/s)

Air

Water

Benzene

78

0.1

1,790

0.23

146

0.09

1 × 10−5

Ethylbenzene

106

0.01

169

0.32

446

0.068

8.5 × 10−6

Toluene

92

0.04

526

0.27

234

0.078

9.2 × 10−6

Xylenes, m-

106

0.01

161

0.29

375

0.068

8.4 × 10−6

Naphthalene

128

1.0 × 10−4

31

0.018

1,544

0.06

8.4 × 10−6

C5–C8 Aliphatics

93

0.1

11

54

2,265

0.08

1 × 10−5

C9–C12 Aliphatics

149

8.7 × 10−4

0.07

65

150,000

0.07

1 × 10−5

C13–C18 Aliphatics

170

1.4 × 10−4

3.5 × 10−4

69

680,000

0.07

5.0 × 10−6

C19–C36 Aliphatics

280

1.1 × 10−6

1.5 × 10−6

110

4.0×10−8

 

 

C9–C10 Aromatics

120

2.9 × 10−3

51

0.33

1,778

0.07

1 × 10−5

C11–C22 Aromatics

150

3.2 × 10−5

5.8

0.03

5,000

0.06

1 × 10−5

1Constants for BTEXN from USEPA RSL guidance (USEPA 2014a); vapor pressures from TOXNET (USNLM 2014); carbon range values from Massachusetts DEP (MADEP 2002b) except C13-C18 Aliphatics (based on EC > 12-16) and C19-C36 Aliphatics (based on EC > 16-35 aliphatics) (TPHCWG 1997)

Petroleum Fuels

“Gasolines”, “middle distillates”, and “residual fuels”, with the middle category including diesel, kerosene, and Stoddard solvent, as defined by the (API 1994), are classifications developed for different mixtures of aliphatic and aromatic compounds.

Gasolines are defined as petroleum mixtures characterized by a predominance of branched alkanes with carbon ranges from C2–C12 and lesser amounts of aromatic compounds (such as BTEX), straight-chain alkanes, cycloalkanes, and alkenes of the same carbon range. Because of the lower molecular weights of these constituents, gasoline has the greatest volatility of the three classes and generally emits the most vapors.

Middle distillates (such as diesel fuel, home heating fuel, kerosene, and jet fuel) are characterized by a wider variety of straight, branched, and cyclic alkanes, as well as PAHs (especially naphthalene and methylnaphthalenes) and heterocyclic compounds with carbon ranges of approximately C5–C9. A small percentage of C8–C25 aliphatic and BTEX compounds are also present in middle distillates. In general, the constituents of middle distillates are less volatile than those of gasolines. Although BTEX compounds are present in middle distillates, their concentrations are several orders of magnitude lower than in gasoline. Naphthalene can also sometimes be considered an aromatic of concern for releases of middle distillates.

Residual fuels (such as fuel oil Nos. 4, 5, and 6, lubricating oils, waste oils, and asphalts) are characterized by complex PAHs and other high-molecular-weight hydrocarbon compounds with carbon ranges that generally fall between C24 and C40. Residual fuels lack a significant amount of volatile compounds and, aside from the potential generation of methane, are generally assumed to pose a minimal VI risk.

Table C-2 summarizes the makeup of past and current gasolines and diesel in terms of commonly targeted, aromatic compounds. Although important in terms of PVI, these compounds constitute a relatively minor proportion of the bulk fuel. The remainder of the fuels is composed of hundreds of nonspecific, hydrocarbon compounds, collectively referred to as TPH. Table C-3 summarizes the typical TPH composition of gasolines and diesel in terms of carbon ranges.

Table C-2. Range of current and past BTEX and naphthalene (BTEXN) concentrations in gasolines and diesel (adapted from Brewer et al. 2013)

Chemical

Gasolines1

Diesel2

Benzene

0.1–3.6%

0.003–0.1%

Ethylbenzene

0.1–3%

0.007–0.2%

Toluene

1–25%

0.007–0.7%

Xylenes

1–15%

0.02–0.5%

Naphthalene

< 1%

0.01–0.8%

1Gasoline ranges (after Potter and Simmons 1998, Kaplan et al. 2007, Weaver et al. 2009)

2Diesel #2 (after Potter and Simmons 1998)

Table C-3. Example carbon range makeup of non-BTEXN, TPH component of gasolines and diesel; exact carbon range makeup of individual fuels will vary (adapted from Brewer et al. 2013)

Carbon range

Gasolines 1

Diesel 1

C5 to C8 aliphatics

45%

< 1%

C9 to C18 aliphatics

12%

35%

C19+ aliphatics

< 1%

43%

C9 to C12+ aromatics

43%

22%

1Indiana Department of Environmental Management (IDEM 2012)

As noted in Table C-3, fuels are dominated by aliphatic compounds, although gasolines can contain a relatively large proportion of C9-C12 aromatic compounds. These aromatic compounds are not significantly volatile and as discussed below play a relatively minor role in PVI. The same is true of the heavier, relatively low-volatility, C9–C18 aliphatic compounds, although in some cases these compounds can make up an important proportion of vapors emitted from middle distillate fuels. Although they constitute a significant proportion of middle distillates and heavy fuels, aliphatic compounds with nineteen or more carbon molecules are not considered volatile and, other than the potential generation of biogenic methane during degradation, do not play a role in PVI.

In addition to gasolines, middle distillates, and residual fuels, other petroleum-related products exhibit potential for PVI, including coal tar, coal tar creosotes, natural gas, and others. Coal tar was created as a by-product of the pyrolysis of coal, coke, or oil in a closed vessel (retort) during production of manufactured gas during the late 1800s to the 1940s (EPRI 1999). Coal tar is a dark reddish brown to black, oily liquid that does not readily mix with water. Coal tar has a strong odor of mothballs or driveway sealer and can be found either as an LNAPL or a DNAPL depending on the formulation process. Coal tar contains a small percentage of VOCs such as BTEX compounds. These compounds are the most soluble in groundwater. Coal tar also contains hundreds of PAHs, which do not readily dissolve in water and are not easily transported in groundwater. With the exception of naphthalene, most of the PAHs also do not readily volatilize. Coal tar creosotes are distilled from coal tar and contain similar compounds with lesser amounts of the heavier compounds (EPRI 1993).

The class of petroleum product released has a significant effect on what COCs could be present and can be used to determine the presence or absence of PVI. The original composition of the product can vary within the classes and can affect the composition of vapors. The amount of BTEX, naphthalene, other aromatics, and aliphatics can differ based on the refinery producing the fuel and the additives in the formulation of the fuels (Table 5-1). For example, in 2011, USEPA restricted the volume of benzene allowed in gasoline, whether refined or imported, to an average of 0.62% volume through exhaust emissions regulations (USEPA 2007a).

Original fuel formulation and composition has likely varied over time and, along with the effects of weathering (noted in Appendix M), can change the composition significantly; therefore, the composition must be evaluated on a site-by-site basis.

Petroleum Vapors

The chemical makeup of vapors emitted from petroleum fuels is predictable based on the composition of the fuels and the theoretical partitioning of chemicals released to soil and groundwater (USEPA 2002c; Hartman 1998). Compounds with comparatively higher vapor pressures and Henry’s law constants can be expected to dominate vapors relative to their proportions in the parent fuels (see Table C-1). Vapors emitted from fresh gasolines can thus be predicted to be dominated by C5–C8 aliphatics (and C2–C4 aliphatics, if present) based both on the relative abundance of these compounds in the parent fuel and on their volatility in comparison to the other compounds. More recent fuels have a lesser amount of benzene because of the increased restriction in the amount of benzene allowed in the fuel; therefore, lower concentrations of benzene in soil vapors can be expected at more recent releases.

Although less volatile than gasolines, diesel and other middle distillate fuels contain variable amounts of C5–C8 aliphatics and a relatively large component of C9–C18 aliphatics (see Table C-2). Therefore, these compounds should again be expected to dominate vapors emitted from soil and groundwater contaminated with these fuels. The relative proportion of C5–C8 to C9–C12 aliphatics will depend in part on the original composition of the fuel (Hayes et al. 2007). The fraction of BTEX in the vapors will be significantly smaller than for gasolines, given their lower relative abundance. Naphthalene could also be present, depending on its presence in the parent fuel.

The USEPA OUST has compiled an empirical database of soil vapor results for more than fifty, primarily gasoline-contaminated sites in the US, Canada, and Australia (USEPA 2013i). The database provides an overview of the basic chemistry of vapors at gasoline release sites. C5–C8 aliphatics overwhelmingly dominate the TPH fraction in samples that were tested.

The predicted composition of vapors from diesel and other middle distillate fuels is also observed in the field. Mixtures of C5–C12 aliphatics composed the overwhelming majority of vapors at sites contaminated with middle distillate fuels (see Brewer et al. 2013 and HDOH 2012 for more details).

In summary, vapors from PHC-contaminated soil and groundwater are generally dominated by volatile, aliphatic compounds with lesser but potentially important amounts of benzene and other aromatic compounds. A detailed discussion of methods to quantitatively assess the vapor intrusion risk posed by individual compounds or groups of compounds, including TPH aliphatics, is beyond the scope of this document. Brewer et al. (2013) present an approach for the quantitative assessment of TPH in vapor intrusion studies and to compare the risk posed by TPH with that of other COCs.

TPH aliphatics can be the primary COCs when benzene is not present or is depleted. The estimated magnitude of this risk depends in part on the toxicity factors applied to individual TPH carbon ranges. Equally important, vapor intrusion depends on the ability of the vapors to migrate though the vadose zone and enter s structure above levels of potential concern. As summarized in the other sections of this document, field studies indicate that biodegradation of hydrocarbon compounds significantly limits this threat at many if not most sites.