Environmental Site Assessment Using Oil Geochemistry

(1) Assessing the origin of oil spills and petroleum soil contamination.

After the Exxon Valdez spill, geochemical analyses (by Exxon) of shoreline oil residues in the Gulf of Alaska revealed that some of the oil residues were not Alaskan crude from the Valdez, but rather were California-derived oil that had been spilled in the Gulf of Alaska at a much earlier date (Bence et al., 1995, 1996). This example illustrates how the origin of an oil spill can be either constrained or pinpointed by sophisticated chemical analyses that distinguish between various oils. (Wang et al, 2006). We utilize several approaches to determine the origin of oil:

• Whole oil gas chromatography, "Oil Fingerprinting"*, of the spilled oil and of all of the produced or pipeline oils in the immediate area of the spill can be used to identify the origin of the spill (Sundararaman and Udo, 1998; Staniloae et al., 2001). This approach requires collection and analysis of all nearby potential source oils for the spill. When a positive correlation of spilled oil and facility oil occurs, then the origin of the spill has been determined. If the spilled oil has undergone weathering, water washing, and/or biodegradation, then biomarker analysis and/or other techniques may be required to identify the spill source, as described below.
• Biomarkers, molecular fossils present in an oil, reflect the type and age of the source rock that generated that oil. Specifically, biomarker distributions in an oil reflect (1) the relative abundance of oil-prone vs. gas-prone organic matter in the source, (2) the source rock age, (3) whether the source was deposited in a marine, lacustrine, fluvio-deltaic or hypersaline setting, (4) whether the source lithology was a carbonate or shale, and (5) the thermal maturity at which the source rock generated that oil (e.g., Peters and Moldowan, 1993). Oils from different basins have different biomarker distributions. Since different potential sources of a spill may involve oil derived from different basins, biomarker distributions can be used to either rule out or rule in potential sources of a spill, and can be used to determine if oil in a contaminated area actually represents more than one spill (Stout et al., 2000, 2001). Biomarkers can also be used to assess the origin of some refined hydrocarbon products (Peters et al., 1992; Stout et al., 2005).
• Polycyclic aromatic hydrocarbons (PAH) are another group of compounds present in oil that are especially useful in identifying the source(s) of a spill. A subset of the PAH in oil are products of the diagenesis of steroid, diterpenoid, triterpenoid and hopanoid biological molecules originally deposited in sediments (biomarkers). Several of these biomarkers are present as fully or partially aromatized compounds with multiple aromatic rings; therefore they are polycyclic aromatic hydrocarbons.  These PAH are resistant to biodegradation conditions typically encountered in spill situations and have proved useful for defining a unique fingerprint characteristic of a given oil; this fingerprint can be used to correlate a biodegraded oil to a sample of its non-degraded equivalent, and hence can be use to identify the source(s) of a petroleum release (e.g., Burns, 1997; Stout et al., 2000, 2001).
• Other (non-biomarker) PAH, such as phenanthrene, alkyl phenathrenes, pyrene, benz(a)anthracene, and similar compounds may undergo biodegradation and weathering during post-spill conditions. Analyses for these compounds are useful during the early stages of an oil spill for source identification (Stout et al., 2000, 2001) and at later stages for determining the extent of weathering and biodegradation and the rates of several of the weathering and biodegradation processes. (Lima et al., 2006; Reddy et al., 2002; Farrington et al., 1982).
• Different oils commonly have different carbon isotopic compositions. Therefore, carbon isotopic analyses of petroleum samples from a contaminated area frequently can be used to constrain the source of the contaminant and to determine if there is more than one source of oil in a contaminated area (e.g., Bence et al., 1996).

(2) Assessing the origin of gas seeps

Natural gas has two primary origins: (1) methane produced by methanogenic bacteria (biogenic gas), and (2) hydrocarbon gas produced by thermal alteration of sedimentary organic matter (thermogenic gas). Thermogenic gas may or may not be co-genetic with oil. Unlike thermogenic gas, biogenic gas is always very dry: it does not contain significant ethane, propane or higher-molecular-weight (i.e., "wet" gases). In addition, biogenic methane contains isotopically lighter carbon (i.e., is more depleted in 13C) than does thermogenic methane. As a result, geochemical analyses can readily reveal if a gas seep represents thermogenic gas, or whether it represents biogenic gas, such as forms from natural degradation of soil organic matter or landfill material (e.g., Coleman et al., 1995; Schoell, 1983, 1984; Schoell et al., 1993).

(3) Monitoring in situ biodegradation of petroleum spills and soil contamination

Microorganisms biodegrade different classes of compounds in petroleum at different rates (e.g., Figure 3.62 in Peters and Moldowan, 1993). As a result, the progressive biodegradation of an oil spill can be monitored by periodic analyses of various compounds in the oil-contaminated soil (e.g., Moldowan et al., 1995; Bence et al, 1996). The early stages of oil biodegradation (loss of paraffins and isoprenoids) can be readily detected by gas chromatographic (GC) analysis of an oil. However, in heavily degraded oils, GC analysis alone cannot distinguish subtle differences in biodegradation due to interference of the unresolved complex mixture (UCM or "hump") that dominates the GC traces. Fortunately, in heavily degraded oils, one can use gas chromatography-mass spectrometry (GC-MS) to quantify the concentrations of biomarkers with differing resistances to biodegradation (e.g., Moldowan and McCaffrey, 1995), allowing the extent of biodegradation to be monitored over time. Recently, the application of GC-GC (comprehensive two-dimensional gas chromatography-gas chromatography) has been shown to be capable of quantitatively resolving many of the compounds in the UCM and providing useful information about weathering and biodegradation, and will certainly prove useful in oil spill "fingerprinting"  (Freisinger and Gaines, 2001; Reddy et al, 2002).  In addition, it has been shown that several of the compounds now resolved from the aromatic hydrocarbon UCM by GC-GC are toxic in laboratory toxicity tests and may have deleterious effects in some oil spill situations (Booth et al, 2007).

In an oil, the quantity of a biomarker that is resistant to biodegradation increases as the oil is biodegraded, because such a compound is "concentrated" in the oil by the loss from the oil of the other less-resistant compounds. Therefore, by comparing the concentration of such a resistant compound in a spill with the concentration of the same compound in the original oil, one can estimate how much of the oil has been degraded. For example, Prince et al. (1994) used the concentration in oil of 17a(H),21b(H)-hopane, a biomarker which is relatively resistant to biodegradation, to estimate the extent of biodegradation of oils.

For more information on the geochemical techniques described here, or to discuss a specific project, e-mail us at info@oiltracers.com, or call us at (214) 584-9169.

* The term "Oil Fingerprinting" came into popular use during the late 1960s and early 1970s with the application of gas chromatography to analyses of spilled oil and potential sources. It was a useful analogy to explain this type of forensic analyses for spilled oil. However, it was recognized then, and remains true today, that the analyses of spilled oils do not have the statistical discriminating power of the human fingerprint in the sense that each human has an individual fingerprint. Analyses of spilled oils and potential sources are usually undertaken by increasingly sophisticated chemical analyses until either all but one potential source oil remains that cannot be distinguished from the spilled oil, or all potential sources have been eliminated and the spill is then a "mystery". The presumption for success using fingerprinting is that a complete collection of possible sources has been secured for the matching analyses. The term "passive tagging" has been used in place of fingerprinting in the past to describe the chemical analyses of oils. The term derives from the process of using the chemicals naturally present in the oil as "tags". The "passive" part of the term was used because there were proposals and some experiments conducted in the late 1960s and early 1970s to introduce "active tags" into various oil cargos to allow for identifying the oils if they were spilled (e.g. see Adlard, 1972; Zafiriou et al, 1973). Various chemicals were proposed as active tags, but the obvious international administrative and logistical effort needed to keep track of such "active tags" prevented operational use of active tagging systems.

References

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