Article pubs.acs.org/est
Unresolved Complex Mixture (UCM) in Coastal Environments Is Derived from Fossil Sources Helen K. White,*,† Li Xu,‡ Paul Hartmann,§ James G. Quinn,∥ and Christopher M. Reddy⊥ †
Department of Chemistry, Haverford College, 370 Lancaster Avenue, Haverford, Pennsylvania 19041, United States National Ocean Sciences Accelerator Mass Spectrometry Facility, Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, United States § Yongsan Education Center, University of Maryland, University College, Unit #15556, APO, AP 96205-5556 ∥ Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882, United States ⊥ Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, United States ‡
ABSTRACT: The unresolved complex mixture (UCM) frequently dominates organic extracts isolated from estuarine and coastal sediments in the vicinity of industrial centers. Despite an obvious link to a petroleum source, speculation exists that biogenic sources also contribute to the UCM. To determine the source of the UCM to these environments, natural abundance radiocarbon (Δ14C) and stable carbon (δ13C) isotopic composition of the UCM solvent-extracted from coastal sediments, road dust, and urban atmospheric particulate in the United States was measured. Extracts of UCM and separate saturate and aromatic fractions from all samples are predominantly (>90%) fossil-derived and hence have a petroleum source. Even the polar fraction of the UCM, which has a Δ14C composition reflecting contributions from recently photosynthesized carbon (−665‰), is composed of ∼66% fossil carbon indicating the presence of petroleum residues that have been transformed into more polar derivatives. The δ13C of the UCM had consistent values (−27.65 ± 0.51‰; n = 16) for all but one sample, indicating a common origin of the UCM. We conclude that in coastal areas dominated by human activities whole fractions of the UCM, as well as separate saturate, aromatic, and polar fractions, are principally derived from petroleum sources.
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may also contribute to the UCM in sediments and aerosols.7−12 The origin of the UCM was first examined using natural radiocarbon (14C) abundance in one sediment sample from Narragansett Bay, which was found to be almost entirely fossil in origin, with no appreciable levels of recently photosynthesized carbon.13 For this measurement, several kilograms of sediment had to be solvent-extracted to obtain enough material for 14C analysis via beta-counting. Since this time, the 14 C content of the UCM has been reported for only one other coastal environment (New York Bight surface sediments14) and one lake environment (Lake Constance, Switzerland lake sediments15). In both cases, the UCM was predominantly fossil-derived. The 14C content of the UCM isolated from noncoastal environments has been measured in hydrothermal sediments from Guaymas Basin where it was found to be identical to the petroleum present in the surrounding sediments.16 While hydrothermal regions such as Guaymas Basin are far from urban harbors and are therefore unlikely major sources of UCM to these environments, hydrocarbon seeps, such as those along the California continental borderland
INTRODUCTION The unresolved complex mixture (UCM) is one of the most abundant, ubiquitous, and understudied classes of solventextractable organic compounds in estuarine and coastal sediments.1 It is not unusual for coastal sediments to contain 10 mg of UCM per gram sediment, whereas polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) are generally near 0.1 and 2 mg per gram sediment, respectively.2,3 The term UCM refers to the presence of a raised baseline in gas chromatograms, which forms a hump-like shape that is composed of unresolved compounds. The first reference to the presence of these unresolved compounds is uncertain, but Blumer et al.4 described chromatograms containing “...a broad unresolved background from cycloparaffins and aromatics...” from the analysis of sediment extracts collected after a No. 2 fuel oil spill. The first use of the term UCM, however, was by Farrington and Quinn5 to describe extracts from sediments from Narragansett Bay (Rhode Island, USA). In the majority of studies to date, the presence of UCM in environmental samples is considered to be an indicator of petroleum pollution.6 While crude oil or petroleum distillates are the assumed source of UCM compounds, studies have suggested that recently photosynthesized carbon or bacterial reworking of naturally derived lipids in situ or during storage © 2012 American Chemical Society
Received: Revised: Accepted: Published: 726
October 15, 2012 November 26, 2012 December 19, 2012 December 19, 2012 dx.doi.org/10.1021/es3042065 | Environ. Sci. Technol. 2013, 47, 726−731
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Table 1. Stable Carbon and Radiocarbon Content of the Solvent-Extracted Unresolved Complex Mixture (UCM) from Sediments and Dust Samples sample ID Boston Harbor, MA 0−2 cm New Bedford Harbor, MA 0−10 cm San Diego Bay, CA 8−10 cm San Francisco Bay, CA 8−10 cm Palos Verde Shelf, CA 0−1 cm Narragansett Bay, RI 0−2 cm Narragansett Bay, RI 0−2 cm Narragansett Bay, RI 0−2 cm Narragansett Bay, RI 0−2 cm Narragansett Bay, RI 0−2 cm Narragansett Bay, RI 0−2 cm Narragansett Bay, RI 0−10 cm Chesapeake Bay, MD 0−10 cm (NIST SRM 1941a) NY/NJ Harbor 0−10 cm (NIST SRM 1944) NY/NJ Harbor 0−10 cm (NIST SRM 1944) NY/NJ Harbor 0−10 cm (NIST SRM 1944) West Falmouth, MA 14−16 cm West Falmouth, MA 14−16 cm Highway Road Dust (RT 102 in RI) Urban Dust (Washington, DC; NIST SRM 1649a) Urban Dust (Washington, DC; NIST SRM 1649a) Urban Dust (Washington, DC; NIST SRM 1649a) Urban Dust (Washington, DC; NIST SRM 1649a) Urban Dust (Washington, DC; NIST SRM 1649a) a
location 42° 19.77′ N 70° 59.57′ W 41° 40.50′ N 70° 54.90′ W 32° 40.50′ N 117° 17.50′ W 37° 47.37′ N 122° 21.59′ W 33° 43.83′ N 122° 24.23′ W 41° 41.66′ N 71° 20.65′ W 41° 43.67′ N 71° 22.02′ W 41° 46.90′ N 71° 22.78′ W 41° 48.71′ N 71° 23.79′ W 41° 51.46′ N 71° 22.55′ W 41° 45.31′ N 71° 23.01′ W 41° 45.68′ N 71° 22.95′ W See NIST (1994)
TOCa (mg g−1 dry wt)
UCM content (μg g−1 dry wt)
δ13C of UCM (‰)
Δ14C of UCM (‰)
f M of UCM
f1*
920
−27.92
−961
0.039
112
f1*
5300
−28.93
−997
0.003
n/a
f1*
55.9
−27.52
−990
0.010
n/a
f1*
10.1
−26.94
−982
0.018
fraction analyzed for 14Cb
16.4
15.3
f1*
210
−27.18
−988
0.012
34.7
f1*
330
−27.69
−969
0.031
40.6
f1*
570
−27.41
−942
0.056
34.7
f1*
760
−27.61
−974
0.026
73.1
f1*
2500
−27.71
−986
0.014
73.2
f1*
2980
−27.65
−989
0.011
83.6
f1*
1890
−27.53
−982
0.018
47
f1 + f 2 *
1660
−27.99
−989
0.011
48
f1 + f 2 *
−27.68
−991
0.009
See NIST (1999)
44
f1*
3200
−28.34
−989
0.004
See NIST (1999)
44
f2
1990
−27.15
−936
0.065
See NIST (1999)
44
f3
1900
−26.89
−665
0.337
41°37.90′ N 70° 38.30′ W 41°37.90′ N 70° 38.30′ W See Reddy and Quinn (1997) See NIST (2001)
117
f1*
6100
−27.14
−982
0.018
117
f2
2800
−25.74
−920
0.081
n/a
f1*
1060
−27.45
−998
0.003
177
f1*
6460
−28.18
−993
0.007
See NIST (2001)
177
f1* T1
711
n/a
−980
0.020
See NIST (2001)
177
f1* T2
807
n/a
−983
0.018
See NIST (2001)
177
f1* T3
427
n/a
−971
0.029
See NIST (2001)
177
f1* T0
4510
n/a
−994
0.007
85.2
Total organic carbon (TOC). bAn asterisk denotes samples that are the nonadducted fraction from urea adduction.
and the Gulf of Mexico, can contribute fossil-derived UCM to the nearshore marine environment.17 This study focuses on the origin of the UCM found in coastal settings, while expanding the geographic spread beyond the two 14C measurements that have been made in nearshore environments since 1973.13,14 Understanding the source of UCM is a critical factor in determining its potential impact on the coastal environment. Most organic compounds present in the UCM have been overlooked and considered harmless, although some components have been shown to pose a significant hazard to benthic
organisms (e.g., 18 and references therein). The quantity of UCM present in sediments is also high enough to significantly increase the amount of total extractable organic matter, and it can comprise 1−20% of the total organic carbon (TOC19). At these concentrations, UCM can promote the formation of anaerobic conditions, which can affect biological community structures, trace metal speciation, and nutrient dynamics. Sediments severely contaminated with petroleum hydrocarbons and UCM may also have a stronger affinity for organic contaminants, causing them to be less bioavailable and hence 727
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with flame ionization detection (GC-FID). The f1 fraction of Urban Dust (Washington, DC; NIST SRM 1649a) sample was also subjected to preparative capillary gas chromatography (PCGC) using an HP 5890 Series II GC system coupled to a Gerstel preparative fraction collector (see 33 for detailed description) so that cuts from the UCM could be taken. Three cuts (T1 [nC20 to nC22 alkanes], T2 [nC24 to nC26 alkanes], and T3 [nC28 to nC30 alkanes]; Figure 1a, b, and c, respectively)
less toxic.20 It has been argued that weathered petroleum and the UCM are the most important sorbents when considering sediment−water partitioning of PCBs, with the former acting as a stronger sorbent than soot or other combustion-derived particles.21 The significance of these impacts is magnified by the fact that the UCM is able to persist in the environment, as seen in two examples of saltmarsh sediments contaminated with No. 2 fuel oil spills at least 30 years after the oil was spilled.22,23 To determine the source of UCM in the nearshore marine environment, the 14C content of the UCM from several coastal environments along the United States, as well as from road dust and from urban atmospheric particulate, has been examined. In addition to examining the saturate and aromatic fractions of the UCM, a polar UCM fraction, as well as subsamples of the saturate UCM fraction, separated by boiling point, have also been examined to assess the heterogeneity of the UCM. These more in-depth analyses have been performed on National Institute of Standards and Technology (NIST) Standard Reference Materials (SRMs) 1649a and 1944, which are commercially available and have the most relevance to the greater scientific community. The majority of other samples in this study were chosen from coastal areas where concentrations of UCM are elevated and therefore more likely to influence the environment in the aforementioned ways.
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EXPERIMENTAL SECTION Samples. Surface sediment samples (up to 10 cm) were collected from various locations over the past two decades (Table 1). The road dust was obtained from the surface of highway Route 102 in Rhode Island.24 Two of the sediment samples and the urban dust are SRMs that are commercially available from the NIST (1994, 1999, 2001). The 14C content of the bulk organic matter, black carbon, and selected polycyclic aromatic hydrocarbons (PAHs) has been described previously for the latter three SRMs.25−27 The 14C content of the bulk organic matter and the organic contaminant concentrations specific to samples from Boston Harbor, New Bedford Harbor, San Diego Bay, Palos Verde Shelf, and West Falmouth have previously been measured1,19,22,28 and PAH distributions in sediment samples collected from Narragansett Bay have also been described elsewhere.1,29 UCM Extraction and Isolation. All samples were extracted by pressurized fluid extraction (as in 22) except samples from Narragansett Bay, which were extracted on a heated-shaker table (as in 29). These two extraction techniques, as well as the subsequent purification and quantification methods, have been shown to produce agreeable data sets.30 Extracts were solvent exchanged into hexane and charged onto a glass column (20 cm × 0.6 cm) packed with fully activated silica gel (100−200 mesh). The column was eluted with 10 mL of hexane to isolate a primarily saturated fraction (trace quantities of alkenes may also be present; f1), 10 mL of dichloromethane to isolate an aromatic fraction (f 2), and 10 mL of dichloromethane and methanol (9:1) to isolate a polar fraction (f 3). For two samples (Narragansett Bay 0−10 cm and Chesapeake Bay 0−10 cm), a combined saturated and aromatic fraction (f1 + f 2) was collected by eluting with only 10 mL of hexane/dichloromethane (1/1). Samples containing saturated compounds (all f1 and f1 + f 2) were urea adducted to remove GC-resolvable long chain n-alkanes,31 which are typically derived from terrestrial plant waxes and have modern carbon signatures (denoted by an asterik in Table 1.32 To characterize the bulk composition of the UCM, all fractions were analyzed by gas chromatography
Figure 1. Preparative capillary gas chromatography cuts of SRM 1649a f1: (a) T1, (b) T2, (c) T3, (d) the remaining extract T0.
were collected in PCGC traps along with the remaining UCM (T0; Figure 1d), which were then passed through silica gel columns and eluted with hexane into precombusted quartz tubes to remove column bleed. The solvent was then evaporated from all quartz tubes under ultrahigh-purity nitrogen gas prior to carbon isotopic analysis. 14 C Analysis and Nomenclature. Extracts were treated and analyzed for stable carbon isotope ratio (δ13C) and radiocarbon abundance (Δ14C) as in White et al.19 Stable carbon isotopic compositions were determined by isotope ratio mass spectrometry (irMS) and 14C content by accelerator mass spectrometry (AMS) at the National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) facility at Woods Hole Oceanographic Institution (WHOI), Woods Hole, MA (as in 34). All 14C measurements are normalized to δ13C values of −25‰ and expressed as either as Δ14C values or the fraction of modern carbon (f M). The former is the per mille (‰) deviation from the international standard for 14C dating, Standard Reference Material 4990B “Oxalic Acid”.35 The f M is derived from the 14C/12C ratio observed in the sample, relative to 0.95 times that of the international standard. In this context, fossil carbon has a Δ14C of −1000‰ (f M = 0), while values at or near 0‰ ( f M = 1) reflect modern levels of 14C. Routine precision for δ13C and Δ14C measurements are ∼0.1 and 10‰, respectively. The Urban Dust (Washington, DC; NIST SRM 1649a) sample cuts (T1, T2, T3, and T0) were submitted as small samples, which have larger Δ 14 C measurement uncertainties of ∼20‰, which is due to a higher statistical error because small targets (90% of the carbon is fossil-derived. This value is calculated using isotope mass balance considering fossil carbon has f M = 0 and recently photosynthesized material (biolipids) present in surface sediments or derived from marine organic matter have f M values ranging from 0.9 to 1.28 It is evident from these analyses that modern biogenic lipids contribute only a small portion of carbon (50% of the mass of petroleum in weathered samples41 and because of their polarity and high abundance, have the potential to be the main contributor to toxicity in the environment.42,43 The polar component of the UCM is therefore of significant environmental relevance and this finding that it is predominantly fossil-derived is a critical step 729
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(12) Silva, T. R.; Lopes, S. R. P.; Spörl, G.; Knoppers, B. A.; Azevedo, D. A. Source characterization using molecular distribution and stable carbon isotopic composition of n-alkanes in sediment cores from the tropical Mundaú-Manguaba estuarine-lagoon system, Brazil. Org. Geochem. 2012, 53, 25−33. (13) Zafiriou, O. C. Petroleum hydrocarbons in Narragansett Bay II. Chemical and isotopic analysis. Est. Coast. Mar. Sci. 1973, 1, 81−87. (14) Farrington, J. W.; Tripp, B. W. Hydrocarbons in Western North Atlantic surface sediments. Geochim. Cosmochim. Acta 1977, 41, 1627− 1641. (15) Giger, W.; Sturm, M.; Sturm, H.; Schaffner, C.; Bonani, G.; Balzer, R.; Hofmann, H. J.; Morenzoni, E.; Nessi, M.; Suter, M.; Wolfli, W. 14C/12C-ratios in organic matter and hydrocarbons extracted from dated Lake sediments. Nucl. Instrum. Methods Phys. Res. 1984, B5, 394−397. (16) Simoneit, B. R. T.; Kvenvolden, K. A. Comparison of 14C ages of hydrothermal petroleums. Org. Geochem. 1994, 21, 525−529. (17) Bauer, J. E.; Spies, R. B.; Vogel, J. S.; Nelson, D. E.; Southon, J. R. Radiocarbon evidence of fossil-carbon cycling in sediments of a nearshore hydrocarbon seep. Nature 1990, 348, 230−232. (18) Booth, A.; Scarlett, A.; Lewis, C. A.; Belt, S. T.; Rowland, S. J. Unresolved Complex Mixtures (UCMs) of Aromatic Hydrocarbons: Branched Alkyl Indanes and Branched Alkyl Tetralins are present in UCMs and accumulated by and toxic to the mussel Mytilus edulis. Environ. Sci. Technol. 2008, 42, 8122−8126. (19) White, H. K.; Reddy, C. M.; Eglinton, T. I. Isotopic constraints on the fate of petroleum residues sequestered in salt marsh sediments. Environ. Sci. Technol. 2005, 39, 2545−2551. (20) Kile, D. E.; Chiou, C. T.; Zhou, Z.; Li, H.; Xu, O. Partition of nonpolar organic pollutants from water to soil and sediment organic matters. Environ. Sci. Technol. 1995, 29, 1401−1406. (21) Jonker, M. T. O.; Barendregt, A. Oil Is a Sedimentary Supersorbent for Polychlorinated Biphenyls. Environ. Sci. Technol. 2006, 40, 3829−3835. (22) Reddy, C. M.; Eglinton, T. I.; Hounshell, A. H.; White, H. K.; Xu, L.; Gaines, R. B.; Frysinger, G. S. The West Falmouth oil spill after thirty years: The persistence of petroleum hydrocarbons in marsh sediments. Environ. Sci. Technol. 2002, 36, 4754−4760. (23) Peacock, E. E.; Hampson, G. R.; Nelson, R. K.; Xu, L.; Frysinger, G. S.; Gaines, R. B.; Farrington, J. W.; Tripp, B. W.; Reddy, C. M. The 1974 spill of the Bouchard 65 oil barge: Petroleum hydrocarbons persist in Winsor Cove salt marsh sediments. Mar. Pollut. Bull. 2007, 54, 214−225. (24) Reddy, C. M.; Quinn, J. G. Environmental chemistry of benzothiazoles derived from rubber. Environ. Sci. Technol. 1997, 31, 2847−2853. (25) National Institute of Standards and Technology. Certificate of Analysis for Standard Reference Material 1941a; Organics in Marine Sediment; Gaithersburg, MD, 1994. (26) National Institute of Standards and Technology. Certificate of Analysis for Standard Reference Material 1944, New York/New Jersey Waterway Sediment; Gaithersburg, MD, 1999. (27) National Institute of Standards and Technology. Certificate of Analysis for Standard Reference Material 1649a, Urban Dust; Gaithersburg, MD, 2001. (28) White, H. K.; Eglinton, T. I.; Reddy, C. M. Radiocarbon-based assessment of fossil fuel-derived contaminant associations in sediments. Environ. Sci. Technol. 2008, 42, 5428−5434. (29) Hartmann, P. C.; Quinn, J. G.; Cairns, R. W.; King, J. W. The distribution and sources of polyclyclic aromatic hyrdrocarbons in Narragansett Bay surface sediments. Mar. Pollut. Bull. 2004, 48, 351− 358. (30) Hartmann, P. C.; Quinn, J. G.; King, J. W.; Tsutsumi, S.; Takada, H. Intercalibration of LABs in Marine Sediment SRM1941a and Their Application as a Molecular Marker in Narragansett Bay Sediments. Environ. Sci. Technol. 2000, 34, 900−906. (31) Marlowe, I. T.; Brassell, S. C.; Eglinton, G.; Green, J. C. Long chain unsaturated ketones and esters in living algae and marine sediments. Org. Geochem. 1984, 6, 135−141.
toward understanding how it can be managed in the environment. It is evident that in coastal areas dominated by human activities or recent spills, the UCM is fossil-derived and does not contain significant contributions from the combustion of wood or other biomass,7,9 or from biological reworking of recently synthesized organic matter as previously suggested.8,11 The latter processes may occur and be present in the more polar fractions of the UCM, but they play a minor role compared to inputs from petroleum. While the more studied, but less abundant organic contaminants such as PCBs and PAHs are regulated, the UCM is not. In light of recent findings about the UCM including its sorbent capability,21 polarity,41 and chronic toxicity,42,43 we must reconsider this important constituent of organic matter and consider its source control, transport, and interplay with other chemical species in the environment.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge John Farrington for his contributions to this project and Cristina Fuller for editorial assistance. Funding for this project was provided by NSF-CHE-008917200 and partial support from the GOMRI DEEP-C consortium.
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