galperin kaplan unusual progression comments 2008cc
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7/31/2019 Galperin Kaplan Unusual Progression Comments 2008CC
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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: ygalperin@aol.com
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).
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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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