age significants c17-pr ratio - discussion
TRANSCRIPT
Age significance ofn-C17/Pr ratios in forensicinvestigations of refinedproduct and crude oilreleases: DiscussionYakov Galperin1 and Isaac R. Kaplan2
We wish to add our opinion to the recent discussion of
themiddle distillate degradation (MDD)model (Hurst
and Schmidt, 2005) in this journal (Oudijk, 2007;Hurst
and Schmidt, 2007). The MDD model was proposed
as an extension of the Christensen and Larsen (1993)
method,which, in addition to hydrocarbon-contaminated
soil, includes separate phase petroleum products and a
broad range of environmental settings. In our opinion,
such an extension contradicts the body of information
accumulated in the literature on biodegradation of pe-
troleum hydrocarbons. The following examples reveal
fundamental flaws of the MDD model.
The first example demonstrates that the rate of
hydrocarbon degradation can change within a single en-
vironmental setting. Figure 1 (Parsons Engineering Sci-
ence, 1999) shows gas chromatograms of soil samples
collected fromdifferent depths at the site impacted by a
single jet fuel JP-4 release that occurred approximately
20 yr ago. These chromatograms clearly demonstrate
that alkane degradation rates are significantly greater in
near-surface samples (most of n-alkane are depleted at
3.5, 5.5, and 7.5 ft [1.06, 1.67, and 2.28 m]) compared
to those at deeper soil samples (n-alkanes are readily
identified at 9.5 and 11 ft [2.89 and 3 m]).
Another example is related to the investigation of the
1979 crude oil spill from a ruptured pipeline in Bemidji,
Minnesota (Hostettler andKvenvolden, 2002).Themain
body of the spilled oil lies on top of the water table in a
shallow sand and gravel aquifer. The analytical data dem-
onstrate that ‘‘the rate of degradation of the aliphatic
hydrocarbons at different sites within the oil body is
variable, most likely related to the availability of water
and oil saturation’’ (Hostettler and Kvenvolden, 2002,
p. 295). Of particular interest are the measured n-C17/
Pr ratio values for samples collected within the spill oil
body at different dates. The initial value of this ratio
(n-C17/Pr)0 is equal to 1.3 (sample BE-31).One sample
collected in 1996 (approximately 17 yr after the spill)
exhibits the same value of 1.3 (sample BE-20), which
attests to a negligible rate of alkane degradation at this
location. A series of samples collected in 1997 and 2001
demonstrate that the alkane degradation rate at the soil-
oil interface is measurably greater (n-C17/Pr = 0) than
that in the body of the spilled oil (n-C17/Pr ratio values
span the range from 0.33 to 1.1).
These two examples clearly demonstrate that the
Hurst and Schmidt (2005) assumption of the uniform
rate of alkane biodegradation, regardless of the site-
specific conditions, is incorrect.
Furthermore, in their attempt to explain the effect
of the initial (n-C17/Pr)0 ratio on the calculated value of
the age of release, Hurst and Schmidt (2005, p. 185)
state that ‘‘to adjust the MDD model regression for
variations in (n-C17/Pr)0 while maintaining the slope at
9.76, we use the relationship between To and (n-C17/
Pr)0 to calculate a new value for To. The value of To
sets the upper limit to the oldest release that can be
dated for a given value of (n-C17/Pr)0. For example, if
(n-C17/Pr)0 of a release is known to be 1.5, To will
equal 14.6 years; for an (n-C17/Pr)0 ratio of 4,To equals
39 years. The latter example is interesting in that it
suggests that the age of hydrocarbon releases grater than
20 years old might be modeled in cases where (n-C17/
Pr)0 of a release exceeds that of the MDD model (i.e.,
2.12; Table 1).’’ This statement is incorrect as written
because it implies that the rate of n-C17 degradation
could depend on the Pr concentration. For example, ac-
cording to Hurst and Schmidt’s (2005) interpretation,
if the initial concentration of n-C17 is 1000 ppm and the
initial concentration of Pr is 472ppm[(n-C17/Pr)0 = 2.2],
the complete degradation of n-C17 will take approx-
imately 20 yr, whereas if the initial concentration of
n-C17 is 1000 ppm but the initial concentration of Pr is
DISCUSSION AND REPLY
Copyright #2008. The American Association of Petroleum Geologists/Division ofEnvironmental Geosciences. All rights reserved.1Environmental Geochemistry Consulting, 13543 Bear Valley Road, Moorpark,California, 93021; [email protected] of California, Los Angeles, 595 Charles Young Drive, Los Angeles,California, 90094; [email protected], IGPP publication No. 6384.
DOI:10.1306/eg.01150807014
Environmental Geosciences, v. 15, no. 2 (June 2008), pp. 85–86 85
250 ppm [(n-C17/Pr)0 = 4], it will take close to 40 yr to
completely degrade n-C17.
Evaluation of the MDD model reveals inconsis-
tencies in its experimental base and theoretical premise.
The linear correlation proposed by Hurst and Schmidt
(2005) between the age of petroleum release and n-C17/
Pr ratio, regardless of the initial product composition and
site-specific conditions, contradicts published analytical
data and the body of information accumulated in the lit-
eratureon thebiodegradationofpetroleumhydrocarbons.
REFERENCES CITED
Christensen, L. B., and T. H. Larsen, 1993, Method for determiningthe age of diesel oil spills in the soil: GroundWater Monitoringand Remediation, Fall, p. 142–149.
Hostettler, F. D., and K. A. Kvenvolden, 2002, Alkylcyclohexanesin environmental geochemistry: Environmental Forensics, v. 3,p. 293–301.
Hurst, R. W., and G. W. Schmidt, 2005, Age significance of nC17/Pr ratios in forensic investigations of refined product and crudeoil releases: Environmental Geosciences, v. 12, no. 3, p. 177–192.
Hurst, R. W., and G.W. Schmidt, 2007, Age significance of nC17/Prratios in forensic investigations of refined product and crude oilreleases: Reply: EnvironmentalGeosciences, v. 14, no. 2, p. 111–112.
Oudijk, G., 2007, Age significance of nC17/Pr ratios in forensicinvestigations of refined product and crude oil releases: Dis-cussion: Environmental Geosciences, v. 14, no. 2, p. 110–111.
Parsons Engineering Science, 1999, Light nonaqueous-phase liquidweathering at various fuel release sites. Report to Air ForceCenter for Environmental Excellence Technology Transfer Divi-sion, Brooks Air Force Base, San Antonio, Texas: http:// www.afcee.brooks.af.mil/products/techtrans/download/fuelweather-ingreport.pdf (accessed September 8, 2006).
Figure 1. JP-4 impacted soilchromatogram results with depth.Defense Fuel Supply Point-Charleston,South Carolina, fuel weatheringstudy (Parsons Engineering Science,1999).
86 Discussion and Reply