Identifying Hydrocarbons

5.1 Major Fuel Alteration in the Environment

Because of the extreme complexity of the composition of petroleum and petroleum-related products, no single analytical method can be used to identify all of its important components in environmental samples. Moreover, because petroleum is a labile mixture, the composition of a product released into the environment begins to change almost immediately because of numerous biochemical and physical processes.

Distribution in the subsurface environment.. As a result of physical processes, hydrocarbon fuel in the subsurface environment is distributed among the following major phases:

  • Fuel vapors as a component of soil gas
  • Fuel absorbed to soil particles
  • Liquid fuel in pore spaces of the soil particles
  • Liquid fuel floating on the groundwater table
  • Fuel dissolved in groundwater

Physical processes. In the soil above the water table, gravity causes liquid fuel to migrate downward from the release source. The infiltrating liquid loses its most volatile components into the soil air and its most soluble constituents to the water surrounding the particles. Fuel components are also lost because of sorption and capillary retention in the vadose zone. When a quantity of the released fuel is larger than losses to volatilization, dissolution and capillary retention, the liquid fuel reaches the water table and begins to accumulate and spread out as a free phase floating product often referred to as non aqueous phase liquid or NAPL (Lyman, et al., 1992).

Weathering. Hydrocarbons released into the environment are also subject to biotic and abiotic weathering reactions in the soil/groundwater media. These processes act in concert, with the rate of transformation being related to the chemical composition of the fuel and local environmental factors, including temperature, soil moisture, nutrient content and oxygen content. Grain size and clay-type are also important parameters for controlling weathering processes in the soil.

Major abiotic reactions include hydrolysis, dehydrogenation, oxidation and polymerization (Lyman, et al., 1992). These reactions are often closely related to microbial (biotic) transformations in the soil profile. Biotic weathering of a hydrocarbon fuel consists of two interdependent mechanisms: microbial uptake (Baughman and Paris, 1981) and metabolic degradation (Singer and Finnerty, 1984). These transformations are likely to occur stepwise, producing alcohols, phenols, aldehydes and carboxylic acids in sequence.

5.2 Analytical Procedures

A variety of geochemical methods for characterization of petroleum, especially those related to oil-oil correlations and the typing of condensates, are often applicable to environmental problems, such as investigation of fugitive fuel spills or natural gas leaks. The analytical procedures used are based on "wet" chemical techniques involving extraction, separation, and pre-concentration, as well as a variety of instrumental methods. These procedures are designed to search for specific characteristics in each sample supplied for analysis. A decision tree based on initial results and available site-specific information, dictates what additional measurements should be made.

The analytical methods commonly used for the forensic characterization of escaped petroleum related products include the following (see also Analyses Sorted by Application ):

  • Gaseous hydrocarbons in the C 1-C 5 range by gas chromatography (GC) with flame ionization detector (FID) (ASTM, 1994, method D2820)
  • Identification of major volatile hydrocarbons in the gasoline range, using high resolution GC/FID (Supelco, 1991) or GC/MS (long column, experimental conditions similar to EPA method 8260B, 1996)
  • Volatile aromatic hydrocarbons, benzene, toluene, ethylbenzene, xylenes (BTEX), using GC with photoionization detector (PID) (USEPA, 1990, method 602/8020) or GC/MS (USEPA 8260B, 1996)
  • Alkyl lead speciation and lead scavengers using GC with electron capture detector (ECD) (experimental conditions are similar to EPA method 608/8080 for PCB analysis, USEPA, 1990)
  • Oxygenated blending agents (alcohols and ethers) by two-dimensional GC/FID (ASTM, 1994, method D4815 for product analysis and modified method D4815 for water and soil analysis) or GC/MS (USEPA 8260B, 1996)
  • Dye additives by thin layer chromatography (TLC) (Touchstone, 1992)
  • Total lead, organic lead and trace metals, especially vanadium (V) and nickel (Ni) using inductively coupled plasma spectrometry (ICP) (USEPA, 1990, method 6010)
  • Simulated distillation by GC/FID (ASTM, 1994, method D2887)
  • n-Alkanes in the C 8-C 35 range, along with specific branched-chain alkanes, using GC/FID or gas chromatography-mass spectrometry (GC/MS) (ASTM, 1994, method D3328)
  • Alkylbenzenes, alkylcyclohexanes, polynuclear aromatic hydrocarbons (PAH) and polycyclic saturated hydrocarbons, such as steranes and terpanes (biomarkers) by GC/MS (experimental conditions are similar to EPA method 625/8270, see also Douglas and Uhler, 1993)
  • Stable isotope ratios for carbon ( 13C/ 12C), hydrogen (D/H), sulfur ( 34S/ 32S) and nitrogen ( 15N/ 14N) by dual collecting isotope ratio mass spectrometry (Hoefs, 1997)

5.3 Fugitive Gases, Their Sources and Differentiation

Natural gas can have two primary origins: one is biogenic (formed by bacteria) and the other is thermogenic (formed by the thermal degradation of organic matter accumulated in the earth’s sedimentary layers). Potentially explosive gases, such as methane, and toxic gases, such as hydrogen sulfide, can be released from soils, drinking water wells, submarine fissures, or abandoned production wells. The nature and source of these gases are important in locations adjacent to landfill sites or at construction sites, such as underground tunnels penetrating organic-rich petroleum source rocks.

Bacterial Origins. Methanogenic bacteria participate with a consortium of other bacteria in the degradation of organic matter under strictly anaerobic conditions in lake, estuarine, and marine sediments, and in water-logged soils. During this process, hydrogen sulfide, carbon dioxide, organic acids, alcohols, ketones, and other compounds are formed by the fermentation and enzymatic action of the bacteria. Where simple organic acids, such as acetic acid are formed, methanogenic bacteria are able to dismutate these acids under highly anoxic conditions into methane by the following fermentation reactions (Conrad, 1993):

CH 3COOH CH 4 + CO 2

Fermentative methane production usually occurs in sanitary landfills and in some organic-rich fresh water environments where organic acids are generated.

In ruminant animals, organic acids are conserved and the formation of methane occurs by the reduction of CO2 with hydrogen gas (which is also a product of fermentation):

CO 2 + 4H 2 CH 4 + H 2O

This mechanism of methane formation is probably the most important in marine and estuarine environments. Bionic gas is dominated by methane and carbon dioxide, in about equal proportions. Traces of ethylene are sometimes measured in biogenic gases.

Thermogenic Methane. When organic matter that accumulates in sedimentary layers is buried, it comes under the influence of the earth’s geothermal gradient, which on the average is ~2.2°C/100 m. Burial to a depth of 2200 m to 2800 m results in the decomposition (cooking) of the organic material. It starts to decompose thermally into methane, associated hydrocarbon gases, carbon dioxide and water at temperatures exceeding about 70°C. Early thermogenic gas contains ethane and other volatile hydrocarbons, but not ethylene.

At high burial temperature (>150°C), methane becomes the dominant or only hydrocarbon gas. Hence, when methane is the only measurable volatile hydrocarbon component of the natural gas, its origin cannot be determined based on chemical composition alone (Kaplan, 1994).

Stable Isotope Ratios. The measurement of the stable isotope ratios of carbon ( 13C/ 12C) and deuterium/hydrogen ( 2H/ 1H or D/H), together with the natural gas composition is an effective method of differentiating sources of methane. The principle of employing stable isotope methods is that the distribution of these isotopes in biologic organic matter is a function of the original photosynthetic fixation of CO 2. Subsequent decomposition of the organic matter follows a kinetic pathway by which the light isotopes ( 12C and 1H) are preferentially selected over the heavy isotopes, Hence, different decomposition products have a different stable isotope distribution (M. Schoell, 1984 and P. D. Jenden & I.R. Kaplan, 1989).

Carbon and hydrogen isotopes are conventionally measured as a ratio of the heavier to the lighter (most abundant) isotope in a gas introduced into a dual collecting mass spectrometer. The separated and purified petroleum product is combusted under vacuum with an oxidizing catalyst to produce CO 2 and H 2O. The carbon dioxide is purified on a vacuum line and then measured in the mass spectrometer for masses 44( 12C 16O 2), 45( 13C 16O 2) and the 13C/ 12C ratio is calculated relative to an international standard.

The water produced during the the combustion of the petroleum hydrocarbons is reacted with activated zinc metal and converted to pure hydrogen gas. The gas is introduced into the mass spectrometer where masses 2( 1H 2) and 3(DH) are measured and a D/H ratio is obtained relative to an international standard. Isotope ratios for carbon can be measured with a precision of ± 0.2‰ and for hydrogen with a precision of ± 2.0‰.

Based on international standards, the Isotope ratios are conventionally expressed as delta values (d) given in per mill (‰) units (Hoefs, 1997) as shown below for carbon. Isotope ratio values are negative if the 13C/ 12C ratio is lower than the standard value (arbitrarily assigned a 0‰) and positive if the 13C/ 12C ratio is greater than the standard value:

The stable carbon isotope ratio of biogenic gases fall in the d 13C range of -45 to -100‰ and a dD range of -150 to -350‰, whereas thermogenic gases cover a d 13C range of -15 to -50‰ and dD of -90 to -300‰ (Schoell, 1988; Whiticar, et al., 1986; Kaplan, 1994). Using this approach, it is possible to differentiate the pathway followed in biogenic methanogenesis from the thermogenic formation of methane, as shown in Figure 1. Therefore, it is generally possible to classify and identify the source of a gas leak by the stable isotope analysis of the gas combined with its compositional gas chromatographic analysis.

Fig. 1 Areas showing characteristic d 13C and dD values for methane generated by biological CO 2 reduction (BIO-R), biological fermentation (BIO-F), and thermogenic processes (THERMO). Shaded areas (MIX) represent methane that can be formed by any processes.

5.4 Gasoline Range Fuel Characterization by Volatile Hydrocarbon Distribution

PIANO (Paraffins, Isoparaffins, Aromatics, Naphthenes, Olefines). The hydrocarbon constituents of gasoline generally span a range from C 3 to C 12 and are conventionally described in terms of their major classes: paraffins, isoparaffins, aromatics, naphthenes, and olefins (PIANO). Relative concentrations of key hydrocarbons in premium (92 octane), mid-grade (89 octane) and regular (87 octane) gasolines, as well as in jet fuel and aviation gasoline, are presented in Table 1.

Table 1. Relative concentrations (%) of gasoline range hydrocarbons of three newly dispensed gasolines, JP-4 and aviation gasoline
Hydrocarbon 87 Octane Gasoline 89 Octane Gasoline 92 Octane Gasoline JP-4 Fuel Aviation Gasoline
1 Propane 0.01 0.10
2 Isobutane 0.14 0.24 0.30 0.01
3 Isobutene 0.04 0.04 0.07
4 Butane/Methanol 0.79 2.18 1.41 2.19
5 trans-2-Butene 0.06 0.06 0.12 0.01
6 cis-2-Butene 0.08 0.09 0.14
7 3-Methyl-1-butene 0.10 0.08 0.07
8 Isopentane 9.47 6.36 5.45 7.12
9 1-Pentene 0.38 0.31 0.25
10 2-Methyl-1-butene 0.59 0.47 0.41
11 Pentane 3.28 2.27 1.86 0.47
12 trans-2-Pentene 0.79 0.70 0.63
13 cis-2-Pentene/t-Butanol 0.46 0.41 0.37
14 2-Methyl-2-butene 1.18 0.95 0.93
15 2,2-Dimethylbutane 0.31 0.20 0.14 0.01 0.23
16 Cyclopentane 0.08 0.05 0.06 0.22
17 2,3-Dimethylbutane/MTBE 2.36 3.51 10.31 0.13 3.68
18 2-Methylpentane 5.72 3.21 3.52 0.79 2.28
19 3-Methylpentane 3.34 2.10 2.00 0.76 1.07
20 Hexane 2.55 1.99 1.56 9.28 0.59
21 trans-2-Hexene 0.48 0.29 0.43
22 3-Methylcyclopentene 0.74 0.30 0.75
23 3-Methyl-2-pentene 0.23 0.16 0.19
24 cis-2-Hexene 0.37 0.30 0.27 0.01
25 3-Methyl-trans-2-pentene 0.10 0.10 0.07 0.18
26 Methylcyclopentane 3.96 2.35 2.03 2.09 0.20
27 2,4-Dimethylpentane 0.91 1.70 0.96 0.36 3.36
28 Benzene 3.15 2.51 2.67 1.07 0.13
29 5-Methyl-1-hexene 0.14 0.13 0.10 0.19
30 Cyclohexane 0.74 0.39 0.28 2.38 0.07
31 2-Methylhexane/TAME 2.49 1.78 1.79 3.22 0.25

Read More: Identifying Hydrocarbons, pg. 2

NEXT: Principles of estimating the date of a Hydrocarbon Fuel Release

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