Determination of microbial carbon sources in petroleum contaminated sediments using molecular 14C analysis

Understanding microbial carbon sources is fundamental to elucidating the role of microbial communities in carbon cycling and in the biodegradation of organic contaminants. Because the majority of anthropogenic contaminants are either directly or indirectly derived from fossil fuels that are devoid of 14C, radiocarbon can be used as a natural inverse tracer of contaminant carbon in the contemporary environment. Here, 14C analysis of individual microbial phospholipid fatty acids (PLFA) was used to characterize the carbon sources utilized bythe active microbial community in salt marsh sediments contaminated by the Florida oil spill of 1969 in Wild Harbor, West Falmouth, MA. A specific goal was to determine whether this community is actively degrading petroleum residues that persist in these sediments. The delta14C values of microbial PLFA in all sediment horizons (contaminated and noncontaminated) matched the delta14C of the total sedimentary organic carbon after petroleum removal, indicating that no measurable metabolism of petroleum residues was occurring. This result agrees with ancillary data such as the delta13C content and distribution of PLFA, and the residual hydrocarbon composition determined by comprehensive two-dimensional gas chromatography (GCxGC) analysis. We hypothesize that microbes have chosen to respire the natural organic matter rather than the residual petroleum hydrocarbons because the former is more labile. Future efforts directed at determining indices of microbial degradation of petroleum hydrocarbons should consider competition with natural organic matter.     

Impact of vegetation on sedimentary organic matter composition and polycyclic aromatic hydrocarbon attenuation

Results from natural and engineered phytoremediation systems provide strong evidencethatvegetated soils mitigate polycyclic aromatic hydrocarbon (PAH) contamination. However, the mechanisms by which PAH mitigation occurs and the impact of plant organic matter on PAH attenuation remain unclear. This study assessed the impact of plant organic matter on PAH attenuation in labile and refractory sediments fractions from a petroleum distillate waste pit that has naturally revegetated. Samples were collected in distinct zones of barren and vegetated areas to assess changes to organic matter composition and PAH content as vegetation colonized and became established in the waste pit. Sediments were fractionated into bulk sediment and humin fractions and analyzed for organic matter composition by isotope ratio mass spectrometry (delta (13)C), 13C nuclear magnetic resonance (13C NMR), delta 14C AMS (accelerator mass spectrometry), and percent organic carbon (%TOC). Gas chromatography mass spectrometry (GC/ MS) of lipid extracts of SOM fractions provided data for PAH distribution histograms, compound weathering ratios, and alkylated and nonalkylated PAH concentrations. Inputs of biogenic plant carbon, PAH weathering, and declines in PAH concentrations are most evidentfor vegetated SOM fractions, particularly humin fractions. Sequestered PAH metabolites were also observed in vegetated humin. These results show that plant organic matter does impact PAH attenuation in both labile and refractory fractions of petroleum distillate waste.     

Radiocarbon-based assessment of fossil fuel-derived contaminant associations in sediments

Hydrophobic organic contaminants (HOCs) are associated with natural organic matter (OM) in the environment via mechanisms such as sorption or chemical binding. The latter interactions are difficult to quantitatively constrain, as HOCs can reside in different OM pools outside of conventional analytical windows. Here, we exploited natural abundance variations in radiocarbon (14C) to trace various fossil fuel-derived HOCs (14C-free) within chemically defined fractions of contemporary OM (modern 14C content) in 13 samples including marine and freshwater sediments and one dust and one soil sample. Samples were sequentially treated by solvent extraction followed by saponification. Radiocarbon analysis of the bulk sample and resulting residues was then performed. Fossil fuel-derived HOCs released by these treatments were quantified from an isotope mass balance approach as well as by gas chromatography-mass spectrometry. For the majority of samples (n = 13), 98-100% of the total HOC pool was solvent extractable. Nonextracted HOCs are only significant (29% of total HOC pool)in one sample containing p,p-2,2-bis(chlorophenyl)-1,1,1-trichloroethane and its metabolites. The infrequency of significant incorporation of HOCs into nonextracted OM residues suggests that most HOCs are mobile and bioavailable in the environment and, as such, have a greater potential to exert adverse effects.     

Influence of sediment-amendment with single-walled carbon nanotubes and diesel soot on bioaccumulation of hydrophobic organic contaminants by benthic invertebrates

Single-walled carbon nanotubes (SWNT) have extremely high affinity for hydrophobic organic contaminants, considerably higher than natural or refractory (e.g., soot and detrital) carbon found in sediments. To evaluate the effect of sediment-associated SWNT on contaminant uptake from sediments by infaunal deposit feeders, we have conducted a comparative bioaccumulation study using two infaunal estuarine invertebrates. The deposit-feeding meiobenthic copepod Amphiascus tenuiremis and the deposit/suspension-feeding polychaete Streblospio benedicti were exposed to hydrophobic organic contaminants (HOCs) including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls, and polybrominated diphenyl ethers for 14 days in the presence of sediment amended with (1) SWNTs, (2) NIST diesel soot, or (3) no carbon amendment. Coaddition of SWNT to sediments significantly reduced bioaccumulation of HOCs in S. benedicti; however, soot addition tended to increase the bioaccumulation of these same compounds in the polychaete worm. Bioaccumulation of HOCs from sediments by copepods (A. tenuiremis) was less dependent on black carbon addition to sediment; neither SWNT nor soot significantly impacted bioaccumulation of PAHs from sediment by this organism. Exposure of both copepods and polychaetes to radiolabeled (14C) SWNT in estuarine sediments revealed that these organisms did not assimilate these materials into their tissues, although S. benedicti did ingest 14C-SWNT, as fecal rods from this organism contained identical 14C activity as that of the sediment to which the worms were exposed.     

Sorption of phenanthrene by nonhydrolyzable organic matter from different size sediments

Nonhydrolyzable organic carbon (NHC) and sorption isotherms of phenanthrene (Phen) on six size-fractionated NHC fractions in two sediments from the Pearl River and Estuary, South China, were investigated. It was found that NHC including ancient organic carbon, black carbon, resistant aquatic organic carbon, and aged soil organic carbon consists mainly of aliphatic and aromatic carbon using 13C nuclear magnetic resonance spectroscopy. The sorption isotherms of Phen by the size-fractionated NHC fractions are nonlinear and are well-fitted to the Freundlich model. For the estuary sediment, the NHC contents and the organic carbon-normalized distribution coefficients (Koc) in the size fractions increase with decreasing particle size. The clay NHC fraction contributes to 70% of the Phen sorption by the bulk NHC isolate. However, for the contaminated river sediment, the NHC contents and the Koc values exhibit no regular variations among the size fractions. The Phen sorption capacities on the size-fractionated NHC fractions of the two sediments are significantly related to their H/C ratios and aliphatic carbon, but negatively to aromatic carbon. The fine-particle NHC fractions with high aliphatic carbon and H/C ratio play a very important role in the sorption, transport, and fate of Phen by the investigated sediments.     

Soot-carbon influenced distribution of PCDD/Fs in the marine environment of the Grenlandsfjords,Norway

The particle associations of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) were studied in both the water column and the surface sediments of a marine fiord system and were found to poorly obey expectations from the organic matter partitioning (OMP) paradigm. The field observations were instead consistent with the presence of a stronger sorbent subdomain such as pyrogenic soot-carbon (SC) playing an important role in affecting the environmental distribution and fate of PCDD/Fs. Solid-water distribution coefficients (Kd) of PCDD/ Fs actually observed in the water column were several orders of magnitude above predictions from a commonly used OMP model. Even when these elevated Kd values were normalized to the particulate organic carbon (POC) content (i.e., K(OC)), the variability in K(OC) for individual PCDD/ Fs at different fjord locations and seasons of factors 100-1,000 suggested that bulk organic matter was not the governing sorbent domain of the suspended particles. Further, POC-normalized particle concentrations of PCDD/ Fs (C(OC)) in a vertical profile (surface water-bottom water-surface sediment) revealed a strong increasing trend with depth. Factors of about 100 higher Coc for all PCDD/Fs in the sediment than in the surface water could not be explained by higher fugacity in the surrounding deep water nor with C:N or delta13C indexes of selective aging of the bulk organic matter. Instead this was hypothesized to reflect selective preservation of a more recalcitrant and highly sorbing, but minor, subdomain such as soot. The extent of enhanced PCDD/F sorption, above the OMP predictions, was positively correlated with the SC:POC ratio of the suspended particles in surface and deep waters. Finally, the geographical distribution of sedimentary PCDD/F concentrations were better explained by the SC content than by the bulk OC content of the sediment. Altogether, these field-based findings add to recent laboratory-based sorption studies to suggest that we need to consider both amorphous OC partitioning domains and SC particles as carriers of planar aromatic contaminants if we are to explain the environmental distribution and fate of pollutants such as PCDD/Fs.     

Sorption to black carbon of organic compounds with varying polarity and planarity

It is becoming increasingly clear that the products of incomplete combustion (soot and charcoal, collectively termed black carbon or BC) can be responsible for as much as 80 - 90% of the total sorption to sediments of aromatic, planar, and hydrophobic compounds such as polycyclic aromatic hydrocarbons or planar polychlorinated biphenyls. In the present study, it was investigated whether a nonpolar aliphatic compound (hexachloroethane) and three nonplanar bipolar compounds with different functional groups [free electron pairs but no aromatic ring (butylate) or free electron pairs and an aromatic ring (diuron, atrazine)] would also show strong and nonlinear sorption to a BC-enriched sediment. At a concentration of 1 ng/L, the extent of elevated BC sorption compared to total organic carbon (TOC) sorption increased in the order atrazine < hexachloroethane < butylate < diuron. Rationalization of the differences between the sorbates was attempted in terms of dispersive and steric effects. This study shows that the effects of strong BC sorption apply to a broader range of organic contaminants than previously thought, and the results will aid in a better understanding of BC sorption mechanisms and improved fate modeling of contaminants in the environment.     

Enhanced pesticide sorption by soils containing particulate matter from crop residue burns

Lack of proper techniques to isolate black carbon (BC) from soils has hindered the understanding of their roles in the sorption and environmental fate of organic contaminants in soils and sediments. The burning of crop residues may be the primary source of BC in agricultural soils. In this study, wheat (Triticum aestivum L.) and rice (Oryza sativa L.) residues were burned, and the resulting particulate matter (ashes) along with a soil were used to sorb diuron from water. Calculations indicated that the burning of crop residues may result in an appreciable level of ashes in soils. The diuron sorption isotherms on ashes were curvilinear Langmuir type, suggestive of surface adsorption and similar to that with activated carbon. Ashes were 400-2500 times more effective than soil in sorbing diuron over the concentration range of 0-6 mg/L. Sorption by wheat ash-amended soils and the degree of isotherm nonlinearity increased with increasing ash content from 0% to 1% (weight), indicating the significant contribution of wheat ash to the sorption. Calculations show that wheat ash and soil independently contributed to the sorption. Above the wheat ash content of 0.05%, the sorption was largely controlled by the ash. Density-based fractionation and repeated HCI-HF washing of wheat ash yielded carbon-enriched fractions and enhanced diuron sorption by these fractions. BC appeared primarily responsible for the high adsorptivity of ashes. Ashes arising from the burning of crop residues may be an important determinant of pesticide immobilization and environmental fate in soils.     

Black carbon and kerogen in soils and sediments. 2. Their roles in equilibrium sorption of less-polar organic pollutants

The first paper of this series reported that soil/sediment organic matter (SOM) can be fractionated into four fractions with a combined wet chemical procedure and that kerogen and black carbon (BC) are major SOM components in soil/sediment samples collected from the industrialized suburban areas of Guangzhou, China. The goal of this study was to determine the sorptive properties forthe four SOM fractions for organic contaminants. Sorption isotherms were measured with a batch technique using phenanthrene and naphthalene as the sorbates and four original and four Soxhlet-extracted soil/sediment samples, 15 isolated SOM fractions, and a char as the sorbents. The results showed that the sorption isotherms measured for all the sorbents were variously nonlinear. The isolated humic acid (HA) exhibited significantly nonlinear sorption, but its contribution to the overall isotherm nonlinearity and sorption capacity of the original soil was insignificant because of its low content in the tested soils and sediments. The particulate kerogen and black carbon (KB) fractions exhibited more nonlinear sorption with much higher organic carbon-normalized capacities for both sorbates. They dominate the observed overall sorption by the tested soils and sediments and are expected to be the most important soil components affecting bioavailability and ultimate fate of hydrophobic organic contaminants (HOCs). The fact that the isolated KB fractions exhibited much higher sorption capacities than when they were associated with soil/sediment matrixes suggested that a large fraction of the particulate kerogen and BC was not accessible to sorbing HOCs. Encapsulation within soil aggregates and surface coverage by inorganic and organic coatings may have caused large variations in the accessibility of fine kerogen and BC particles to HOCs and hence lowered the sorption capacity of the soil. This variability posts an ultimate challenge for precisely predicting HOC sorption by soils from the contents of different types of SOM.     

The West Falmouth oil spill after thirty years: the persistence of petroleum hydrocarbons in marsh sediments

The long-term fate of petroleum hydrocarbons in marsh sediments (West Falmouth, MA) contaminated in 1969 by the spill of the barge Florida was investigated. A 36-cm-long sediment core was collected in August 2000, and sediment extracts were analyzed by gas chromatography (GC) and comprehensive two-dimensional gas chromatography (GC x GC). The latter technique is capable of separating 1 order of magnitude more compounds than the former and was used to observe whether any compositional changes in the unresolved complex mixture (UCM) occurred. No evidence of petroleum residues was detected in the top 6 cm (0-6 cm) and the lower 8 cm (28-36 cm) of the core. However, the central sections 16-28 cm) were dominated by a UCM in the boiling range of n-C13-n-C25 alkanes, consistent with a No. 2 fuel oil source. The 12-14- and 14-16-cm sections had the highest concentrations of UCM approximately 8 mg g(-1)). These values are similar to concentrations observed shortly after the spill. Initial GC x GC analysis revealed that only the n-alkanes were completely degraded, and contrary to previous studies, pristane and phytane as well as numerous other branched alkanes are still present in the sediments. These results suggestthatatthis site hydrocarbon contamination will persist indefinitely in the sedimentary record.     

Potential for plastics to transport hydrophobic contaminants

Plastic debris litters marine and terrestrial habitats worldwide. It is ingested by numerous species of animals, causing deleterious physical effects. High concentrations of hydrophobic organic contaminants have also been measured on plastic debris collected from the environment, but the fate of these contaminants is poorly understood. Here, we examine the uptake and subsequent release of phenanthrene by three plastics. Equilibrium distribution coefficients for sorption of phenanthrene from seawater onto the plastics varied by more than an order of magnitude (polyethylene > polypropylene > polyvinyl chloride (PVC)). In all cases, sorption to plastics greatly exceeded sorption to two natural sediments. Desorption rates of phenanthrene from the plastics or sediments back into solution spanned several orders of magnitude. As expected, desorption occurred more rapidly from the sediments than from the plastics. Using the equilibrium partitioning method, the effects of adding very small quantities of plastic with sorbed phenanthrene to sediment inhabited by the lugworm (Arenicola marina) were evaluated. We estimate that the addition of as little as 1 microg of contaminated polyethylene to a gram of sediment would give a significant increase in phenanthrene accumulation by A. marina. Thus, plastics may be important agents in the transport of hydrophobic contaminants to sediment-dwelling organisms.     

Sorption of perfluorinated surfactants on sediments

The sorption of anionic perfluorochemical (PFC) surfactants of varying chain lengths to sediments was investigated using natural sediments of varying iron oxide and organic carbon content. Three classes of PFC surfactants were evaluated for sorptive potential: perfluorocarboxylates, perfluorosulfonates, and perfluorooctyl sulfonamide acetic acids. PFC surfactant sorption was influenced by both sediment-specific and solution-specific parameters. Sediment organic carbon, rather than sediment iron oxide content, was the dominant sediment-parameter affecting sorption, indicating the importance of hydrophobic interactions. However, sorption also increased with increasing solution [Ca2+] and decreasing pH, suggesting that electrostatic interactions play a role. Perfluorocarbon chain length was the dominant structural feature influencing sorption, with each CF2 moiety contributing 0.50-0.60 log units to the measured distribution coefficients. The sulfonate moiety contributed an additional 0.23 log units to the measured distribution coefficient, when compared to carboxylate analogs. In addition, the perfluorooctyl sulfonamide acetic acids demonstrated substantially stronger sorption than perfluorooctane sulfonate (PFOS). These data should prove useful for modeling the environmental fate of this class of contaminants.     

Compound-specific carbon and hydrogen isotope analysis of sub-parts per billion level waterborne petroleum hydrocarbons

Compound-specific carbon and hydrogen isotope analysis (CSCIA and CSHIA) has been increasingly used to study the source, transport, and bioremediation of organic contaminants such as petroleum hydrocarbons. In natural aquatic systems, dissolved contaminants represent the bioavailable fraction that generally is of the greatest toxicological significance. However, determining the isotopic ratios of waterborne hydrophobic contaminants in natural waters is very challenging because of their extremely low concentrations (often at sub-parts ber billion, or even lower). To acquire sufficient quantities of polycyclic aromatic hydrocarbons with 10 ng/L concentration for CSHIA, more than 1000 L of water must be extracted. Conventional liquid/liquid or solid-phase extraction is not suitable for such large volume extractions. We have developed a new approach that is capable of efficiently sampling sub-parts per billion level waterborne petroleum hydrocarbons for CSIA. We use semipermeable membrane devices (SPMDs) to accumulate hydrophobic contaminants from polluted waters and then recover the compounds in the laboratory for CSIA. In this study, we demonstrate, under a variety of experimental conditions (different concentrations, temperatures, and turbulence levels), that SPMD-associated processes do not induce C and H isotopic fractionations. The applicability of SPMD-CSIA technology to natural systems is further demonstrated by determining the delta13C and deltaD values of petroleum hydrocarbons present in the Pawtuxet River, RI. Our results show that the combined SPMD-CSIA is an effective tool to investigate the source and fate of hydrophobic contaminants in the aquatic environments.     

Black carbon and kerogen in soils and sediments. 1. Quantification and characterization

A comprehensive wet chemical procedure was developed by combining acid demineralization, base extraction, and dichromate oxidation for fractionation and quantitative isolation of soil/sediment organic matter (SOM) into four fractions: (1) humic acids + kerogen + BC (HKB); (2) kerogen + BC (KB); (3) humic acid (HA); and (4) BC. The soil/sediment samples tested were collected from the suburban areas of Guangzhou, a rapidly developing city of China. The results show that BC and kerogen constitute 57.8-80.6% of the total organic carbon (TOC) and that the relative content of BC ranges from 18.3% to 41.0% of the TOC, indicating that both BC and kerogen are major organic components in soils and sediments from this industrialized region. Systematic characterization of the isolated SOMs shows that both BC and kerogen have sizes ranging from a few microns to above 100 microm, relatively low O/C and H/C atomic ratios, and low contents of oxygen-containing functional groups. The isolated BC has unique fusinite and semifusinite macerals, highly porous nature, and structures indicative of its possible origins. The study indicates that SOM is highly heterogeneous and that humin, the nonextractable humus fraction, consists mainly of kerogen and BC materials in the tested soil/sediment samples. The presence of these materials in soils and sediments may have significant impacts on pollutant mass transfer and transformation processes such as desorption and bioavailability of less polar organic chemicals in surface aquatic and groundwater environments.     

Improving predictability of sediment-porewater partitioning models using trends observed with PCB-contaminated field sediments

More than 1900 sediment-water partitioning coefficients were measured for 58 polychlorinated biphenyl (PCB) congeners in 53 historically contaminated sediments collected from 10 urban and rural waterways in the United States and Canada. Freely dissolved porewater concentrations were determined using passive sampling with polyoxymethylene. Measured total organic carbon (TOC)/water partitioning coefficients, K(TOC), ranged from one to nearly three orders-of-magnitude higher than typical literature values based on spiking experiments and model predictions. Although total PCB concentrations ranged from 0.08 to 194 mg/kg, the more highly contaminated sediments showed only slightly lower K(TOC) values than less-contaminated sediments. No correlation was observed between log K(TOC) values and sediment TOC, black carbon (BC), or BC/TOC fractions (r(2) typically <0.1). Utilizing a two-carbon model incorporating anthropogenic BC did not improve predictions over a one-carbon TOC model. A comparison of models recently validated for field data showed that a coal-tar poly parameter linear-free energy relationship (PP-LFER) and a Raoult's Law model were successful at predicting average log K(TOC) values, without the need for any calibration or fitting (within a factor of 10 more than 90% of the time, and within a factor of 30 more than 99% of the time). Predictions were further improved by the introduction of a Weathering Factor (WF) that accounts for the relative depletion of lower molecular weight congeners due to weathering. Highly weathered sediments (with a WF near 1) tended to follow the coal-tar PP-LFER and Raoult's Law model the closest. Less-weathered sediments (with WF ? 1) sorbed less than predicted by these models. Noncalibrated WF inclusive coal-tar PP-LFER and Raoult's Law models performed as well or better than a quantitative-structure activity relationship (QSAR) model calibrated specifically to the data. These recommended partitioning models here can readily be used for all 209-PCB congeners.     

Strong sorption of phenanthrene by condensed organic matter in soils and sediments

The nonhydrolyzable carbon (NHC) and black carbon (BC) in three contaminated soils and seven sediments from the Pearl River Delta and Estuary, China, were isolated upon treatments with an acid hydrolysis method and with a combustion method at 375 degrees C, respectively, and their sorption isotherms for phenanthrene (Phen) were established. It was found that NHC is chemically and structurally different from the biopolymer and humic substances and consists mainly of aliphatic and aromatic carbon using elemental analysis, 13C nuclear magnetic resonance spectroscopy (13C NMR), and Fourier transformed infrared spectroscopy (FTIR). All the sorption isotherms are nonlinear and are well fitted by the Freundlich model. The single-point organic carbon-normalized distribution coefficient (K(oc)) measured for the isolated NHC is 1.3-7.7 times higher than that for the bulk samples at the same aqueous concentration of Phen. The NHC fractions play a dominant role to the overall sorption in the bulk samples. The bulk soils and their NHC fractions have lower sorption capacity than the bulk sediments and their NHC fractions, relating to the different source of organic matter between soils and sediments. The Phen sorption capacity in the NHC samples is related significantlyto H/C ratios and aliphatic carbon, but negatively to aromatic carbon, demonstrating the important role of aliphatic carbon to the Phen sorption and the fate in the investigated soils and sediments.     

Monitoring biodegradation of methyl tert-butyl ether (MTBE) using compound-specific carbon isotope analysis

Methyl tert-butyl ether (MTBE), the most common gasoline oxygenate, is frequently detected in surface water and groundwater. The aim of this study was to evaluate the potential of compound-specific isotope analysis to assess in situ biodegradation of MTBE in groundwater. For that purpose, the effect of relevant physical and biological processes on carbon isotope ratios of MTBE was evaluated in laboratory studies. Carbon isotope fractionation during organic phase/gas-phase partitioning (0.50 +/- 0.15@1000), aqueous phase/gas-phase partitioning (0.17 +/- 0.05@1000), and organic phase/aqueous-phase partitioning (0.18 +/- 0.24@1000) was small in comparison to carbon isotope fractionation measured during biodegradation of MTBE in microcosms based on aquifer sediments of the Borden site. In experiments with MTBE as the only substrate and a cometabolic experiment with 3-methypentane as primary substrate, MTBE became enriched in 13C by 5.1 to 6.9@1000 after 95 to 97% degradation. For both experiments, similar isotopic enrichment factors were obtained (-1.52 +/- 0.06 to -1.97 +/- 0.05@1000). Biodegradation of TBA, which accumulated transiently in the cometabolic microcosms, was also accompanied by carbon isotope fractionation, with an isotopic enrichment factor of -4.21 +/- 0.07@1000. This study suggests that carbon isotope analysis is a potential tool to trace in situ biodegradation of MTBE and TBA and thus to better understand the fate of these contaminants in the environment.     

Long-term recovery of PCB-contaminated surface sediments at the Sangamo-westonl Twelvemile Creek/lake Hartwell Superfund Site

Natural recovery of contaminated sediments relies on burial of contaminated sediments with increasingly clean sediments over time (i.e., natural capping). Natural capping reduces the risk of resuspension of contaminated surface sediments, and it reduces the potential for contaminant transport into the food chain by limiting bioturbation of contaminated surface or near-surface sediments. This study evaluated the natural recovery of surface sediments contaminated with polychlorinated biphenyls (PCBs) at the Sangamo-Weston/Twelvemile Creek/Lake Hartwell Superfund Site (Lake Hartwell), Pickens County, SC. The primary focus was on sediment recovery resulting from natural capping processes. Total PCB (t-PCB), lead-210 (210Pb), and cesium-137 (137Cs) sediment core profiles were used to establish vertical t-PCB concentration profiles, age date sediments, and determine surface sedimentation and surface sediment recovery rates in 18 cores collected along 10 transects. Four upgradient transects in the headwaters of Lake Hartwell were impacted by historical sediment releases from three upgradient sediment impoundments. These transects were characterized by silt/ clay and sand layering. The highest PCB concentrations were associated with silt/clay layers (1.8-3.5% total organic carbon (TOC)), while sand layers (0.05-0.32% TOC) contained much lower PCB concentrations. The historical sediment releases resulted in substantial burial of PCB-contaminated sediment in the vicinity of these four cores; each core contained less than 1 mg/kg t-PCBs in the surface sand layers. Cores collected from six downgradient Lake Hartwell transects consisted primarily of silt and clay (0.91-5.1% TOC) and were less noticeably impacted by the release of sand from the impoundments. Vertical t-PCB concentration profiles in these cores began with relatively low PCB concentrations at the sediment-water interface and increased in concentration with depth until maximum PCB concentrations were measured at approximately 30-60 cm below the sediment-water interface, ca. 1960-1980. Maximum t-PCB concentrations were followed by progressively decreasing concentrations with depth until the t-PCB concentrations approached the detection limit, where sediments were likely deposited before the onset of PCB use at the Sangamo-Weston plant. The sediments containing the maximum PCB concentrations are associated with the period of maximum PCB release into the watershed. Sedimentation rates averaged 2.1 +/- 1.5 g/(cm2 yr) for 12 of 18 cores collected. The 1994 Record of Decision cleanup requirement is 1.0 mg/kg; two more goals (0.4 and 0.05 mg/kg t-PCBs) also were identified. Average surface sedimentation requirements to meet the three goals were 1.4 +/- 3.7, 11 +/- 4.2, and 33 +/- 11 cm, respectively. Using the age dating results, the average recovery dates to meet these goals were 2000.6 +/- 2.7, 2007.4 +/- 3.5, and 2022.7 +/- 11 yr, respectively. (The 95% prediction limits for these values also are provided.) Despite the reduction in surface sediment PCB concentrations, PCB concentrations measured in largemouth bass and hybrid bass filets continue to exceed the 2.0 mg/kg FDA fish tolerance level.     

Level and degradation of Deepwater Horizon spilled oil in coastal marsh sediments and pore-water

This research investigates the level and degradation of oil at ten selected Gulf saltmarsh sites months after the 2010 BP Macondo-1 well oil spill. Very high levels (10-28%) of organic carbon within the heavily oiled sediments are clearly distinguished from those in pristine sediments (<3%). Dissolved organic carbon in contaminated pore-waters, ranging up to hundreds of mg/kg, are 1 to 2 orders of magnitude higher than those at pristine sites. Heavily oiled sediments are characterized by very high sulfide concentrations (up to 80 mg/kg) and abundance of sulfate reducing bacteria. Geochemical biomarkers and stable carbon isotope analyses fingerprint the presence of oils in sediments. Ratios of selected parameters calculated from the gas chromatograph spectra are in a remarkable narrow range among spilled oils and initial BP crude. At oiled sites dominated by C(4) plants, δ(13)C values of sediments (-20.8 ± 2.0‰) have been shifted significantly lower compared to marsh plants (-14.8 ± 0.6‰) due to the inflow of isotopically lighter oil (-27 ± 0.2‰). Our results show that (1) lighter compounds of oil are quickly degraded by microbes while the heavier fractions of oil still remain and (2) higher inputs of organic matter from the oil spill enhance the key microbial processes associated with sulfate reducing bacteria.     

Spatial and Temporal Assessment of Mercury and Organic Matter in Thermokarst Affected Lakes of the Mackenzie Delta Uplands, NT, Canada

We examined dated sediment cores from 14 thermokarst affected lakes in the Mackenzie Delta uplands, NT, Arctic Canada, using a case-control analysis to determine how retrogressive thaw slump development from degrading permafrost affected the delivery of mercury (Hg) and organic carbon (OC) to lakes. We show that sediments from the lakes with retrogressive thaw slump development on their shorelines (slump-affected lakes) had higher sedimentation rates and lower total Hg (THg), methyl mercury (MeHg), and lower organic carbon concentrations compared to lakes where thaw slumps were absent (reference lakes). There was no difference in focus-corrected Hg flux to sediments between reference lakes and slump-affected lakes, indicating that the lower sediment Hg concentration in slump-affected lakes was due to dilution by rapid inorganic sedimentation in the slump-affected lakes. Sedimentation rates were inversely correlated with THg concentrations in sediments among the 14 lakes considered, and explained 68% of the variance in THg concentration in surface sediment, further supporting the dilution hypothesis. We observed higher S2 (algal-derived carbon) and particulate organic carbon (POC) concentrations in sediment profiles from reference lakes than in slump lakes, likely because of dilution by inorganic siliciclastic matter in cores from slump-affected lakes. We conclude that retrogressive thaw slump development increases inorganic sedimentation in lakes, and decreases concentrations of organic carbon and associated Hg and MeHg in sediments.