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Environmental Forensics, 9:117120, 2008

Copyright C Taylor & Francis Group, LLC

ISSN: 15275922 print / 15275930 online

DOI: 10.1080/15275920802115324

Commentary

Comments on the Reported Unusual Progression of PetroleumHydrocarbon Distribution Patterns DuringEnvironmental Weathering

Yakov Galperin1 and Isaac R. Kaplan2

1Environmental Geochemistry Consulting, Moorpark, CA, USA2Department of Earth & Space Sciences and Institute of Geophysics & Planetary Physics, University of California, Los Angeles,

CA, USA

Keywords: hydrocarbon fingerprinting, petroleum degradation, methanogenic biodegradation, alkane degradation

Over the past 5 years, a group of United States Geological Sur-

vey (USGS) researchers published a series of articles (Hostet-

tler and Kvenvolden, 2002; Bekins et al., 2005; Hostettler et

al., 2007) suggesting a rather unusual progression of petroleum

degradation in the subsurface environment under anoxic (pre-

dominantly methanogenic) conditions. Based on results of the

two field studies, the researchers concluded that for the >C10petroleum fraction, degradation under methanogenic condi-

tions depletes the longer chain normal alkanes (n-alkanes), n-

alkylcyclohexanes andn-alkylbenzenes before the shorter chain

homologs. Moreover, Hostettler et al. (2007) stated that n-

alkyl[-substituted] compounds are apparently attacked at the end

of n-alkane chains, resulting in the formation of progressively

lower MW homologs. . . (p. 152). USGS researchers warned

thatif unrecognizedthe effect of this process on a degraded

petroleum product fingerprint could be erroneously attributed to

admixture of a lower boiling petroleum product (Hostettler and

Kvenvolden, 2002).

Whereas preferential methanogenic degradation of heaviern-alkanes was reported for gasoline range (C10) homologs, or biogenic formation of

lower-molecular-weight n-alkanes by breaking down higher-

molecular-weight n-alkane homologs. Apart from the previously

Received 14 November 2007; accepted 1 December 2007.Address correspondence to Yakov Galperin, Environmental Geo-

chemistry Consulting, 13543 Bear Valley Road, Moorpark, CA 93021.E-mail: [email protected]

referenced USGS publications, other publications that address

the issue of weathering by biodegradation support a degradation

progression with preferential depletion of lighter members of

n-alkane homologues series (Wilkes et al., 1995, 2007; Swan-

nell et al., 1996; Grishchenkov et al., 2000; Widdel and Rabus,

2001; Artz et al., 2002; Townsend et al., 2003; Davidova et al.,

2005). Our literature search disclosed that photooxidation is theonly environmental degradation process that reportedly gener-

ates lower-molecular-weight n-alkanes (Dutta and Harayama,

2000).

Considering the current status of scientific knowledge con-

cerning the mechanism of methanogenic transformation of

n-alkanes, n-alkylcyclohexanes, and n-alkylbenzenes (Widdel

and Rabus, 2001; Gieg and Suflita, 2002; Young and Phelps,

2005), the degradation progression proposed by the USGS re-

searchers should be considered a hypothesis until confirmed

or invalidated by additional studies. However, they proceeded

to offer this untested hypothesis as a decisive argument in a

multimillion-dollar environmental litigation (Hostettler et al.,2007).

Results of the laboratory incubation experiment published by

the USGS group (Hostettler et al., 2007) seem to provide some

degree of support to their hypothesis forn-alkanes degradation

progression under methanogenic conditions. However, results of

an independentlaboratory studyof methanogenic degradationof

alkanes in crude oil added to anaerobic microcosms inoculated

with estuarine sediment (Aitken et al., 2007) contradicts the

USGS group results and displays a conventional progression

that shows no relative enrichment of the lower-molecular-weight

n-alkanes (Figure 1).

117


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118 Y. Galperin and I. R. Kaplan

Figure 1. Gas chromatograms of A) undegraded North Sea crude oil and B) same crude oil after 15 months laboratory degradation under methanogenicconditions (modified from Aitken et al., 2007).

The following discussion concerns two case studies that

Hostettler and her colleagues relied on in formulating the hy-

pothesis of the unique pattern of petroleum degradation under

methanogenic condition. The first case represents a long-term

diesel fuel spill in Mandan, North Dakota. Based on their inves-

tigation, USGS researchers concluded that the observed shifttowards the lighter hydrocarbons in the maximum of n-alkanes

and n-alkylcyclohexanes distribution patterns was due to the

unique methanogenic progression of the fuel degradation as de-

scribed previously (Hostettler and Kvenvolden, 2002). From the

onset of the investigation, Hostettler and Kvenvolden (2002) as-

sumed that the only documented spill component is diesel fuel

from the railway yard in the downtown area (p. 295). However,

the site has had a prolonged history of multiple releases at dif-

ferent locations (Roberts, 2001), which suggests variability in

chemical composition of the diesel fuel released. In addition, a

study performed by the independent group of researchers iden-

tified a source of a lighter petroleum product, kerosene, located

Figure2. Distribution pattern ofn-alkanes in sample BE-31(modifiedfrom

Hostettler et al., 2007). Pr, pristane.

upgradient from the diesel fuel plume (Stout et al., 2006; Stout

and Uhler, 2006). This second group then concluded that the ob-

served changes in hydrocarbon fingerprints could be explained

by a mixing of diesel and kerosene fuel plumes.

The second case is the August 20, 1979, accidental crude

oil spill from a ruptured pipeline in the vicinity of Bemidji,Minnesota. Among the main reasons for selecting this site for

a long-term study USGS listed the introduction of a source

of uniform composition at a known place and time (USGS,

2006). Although USGS researchers claim that this oil release is

well documented and characterized, their investigation at the site

began in May 1983, nearly 4 years after the accident and after an

extensive recovery effort by the pipeline company. Whereas ma-

jor details of this recovery effort appear to be well documented

(Hult, 1984), there is a good possibility that some of the recovery

procedures could have caused pronounced changes in the chem-

ical composition of the crude oil released. It is also noteworthy

that the rupture of a shallow, high-pressure pipeline has resultedin a large spray zone covering approximately 7500 m2 of land

surface. For at least 1 month, the oil released on the ground was

exposed to the atmosphere, which undoubtedly caused changes

in itschemicalcomposition. Theoil originally pooledat a few to-

pographic depressions and,after approximately 70% of the crude

oil was recovered by pumping from surface pools, trenching and

excavation, or surface burning, the residual oil percolated down

to the groundwater, forming a separate-phase hydrocarbon layer

on the water table. It is likely that due to the differences in time

of the surface exposure and migration pathways, even at the very

beginning of the subsurface degradation period, residual crude

oil contamination at different locations in the vadose zone and


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Comment: Hydrocarbons in Methanogenic Environment 119

Figure 3. Distribution pattern ofn-alkanes in fresh Alberta crude oil (modified from Wang et al., 2003).

Figure 4. Distribution pattern ofn-alkanes in evaporated (36.8% wt.) Alberta crude oil (modified from Wang et al., 2003).

separate-phase crude oil floating on the water table could have

variable degrees of chemical alteration compared with the initial

crude oil composition. This heterogeneity of crude oil chemical

composition at the initial stages of subsurface degradation has

not been accounted for in the USGS researchers interpretation

of analytical data.

For the Bemidji oil spill case, the observed changes in dis-

tribution patterns of major homologs series of hydrocarbons

were reportedly compared with the corresponding patterns in a

sample of, archived original crude oil (sample BE-31; How-

ever, the n-alkane profile of this sample (Figure 2; Hostettler

et al., 2007) does not seem to corroborate this claim. A re-

duced abundance of light-end alkanes in this sample is not

typical of unaltered Alberta crude oil that was released at the

site. A published n-alkane profile of unaltered Alberta crude

oil presented in Figure 3 (Wang et al., 2003) exhibits an un-

mistakably different distribution with a much higher content of

the light-end hydrocarbons. The difference may be due to some

kind of environmental alteration of crude oil in reference sample

BE-31.

The n-alkane profile of sample BE-31 (Figure 2) appears

to exhibit similarity with that of a weathered by evaporation

(36.8% weight loss) Alberta crude oil shown in Figure 4 (Wang

et al., 2003). This observation suggests that reference oil sam-

ple BE-31 represents crude oil that has been already altered by a

weathering process (e.g., evaporation) and therefore may be in-

valid as a reference standard for the composition of the released

unaltered crude oil.

This brief discussion provides a basis for the alternative inter-

pretation of analytical data obtained at Mandan, North Dakota,

and Bemidji, Minnesota, sites. Currently, the proposed progres-

sion of petroleum hydrocarbon distribution patterns by USGS

researchers can be considered as an interesting hypothesis that

requires a careful evaluation.

In conclusion, we would like to point out an obvious misrep-

resentation by USGS group of the previously published stud-

ies. According to Hostettler et al. (2007), they introduced this

unusual progression of petroleum degradation as opposed to a

widely accepted degradation sequence in aerobic environment

described by Kaplan et al. (1997) and Wang et al. (1998).

However, these latter publications discuss cumulative changes

in chemical composition of petroleum products resulting from

physical and biochemical weathering in a typical subsurface en-

vironment, without referring to specific redox conditions.

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