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. Author manuscript; available in PMC: 2022 Feb 1.
Published in final edited form as: Mar Pollut Bull. 2020 Dec 18;163:111941. doi: 10.1016/j.marpolbul.2020.111941

Toxicity of Sediment Oiled with Diluted Bitumens to Freshwater and Estuarine Amphipods

MG Barron 1,*, EM Moso 1, RN Conmy 2, P Meyer 3, D Sundaravadivelu 4
PMCID: PMC8201642  NIHMSID: NIHMS1690348  PMID: 33348288

Abstract

To address knowledge gaps and the lack of benchmarks on the toxicity of dilbit oiled sediments, weathered Cold Lake Blend (CLB) and Western Canadian Select (WCS) were assessed in 10-day sediment tests with the amphipods Hyalella azteca and Leptocheirus plumulosus. Lowest observed effect concentrations (LOECs) and 20% effect levels (EC20s) were determined for wet weight sediment concentrations of TPH and total PAHs normalized to 1% organic carbon. LOECs and EC20s for TPH ranged from 216 to 1165 mg/kg sediment in H. azteca, and from 64 to 75 mg/kg sediment in L. plumulosus. Dilbit LOECs and EC20s for total PAHs ranged from 2.9 to 11.8 mg/kg sediment in H. azteca, and from 0.75 to 0.87 mg/kg in L. plumulosus. Comparison of toxicity-based benchmarks derived from the current study to sediment concentrations from past spills indicate that dilbit spills in aquatic habitats may pose substantial risks to freshwater and estuarine benthic organisms.

INTRODUCTION

Diluted bitumens (dilbits) are heavy crude oils produced from blending the highly asphaltic bitumens from the Canadian oil sands with lighter petroleum products. Dilbits are increasingly transported within North America and spills have occurred in freshwater and marine environments (Stantec 2012; USEPA 2016; Dew et al. 2015; Walker 2016; NAS 2016; Stoyanovich et al. 2019). For example, the July 2010 spill of dilbit into the Kalamazoo River Michigan was the largest freshwater spill of any oil type and one of the costliest in U.S. history (Lee et al. 2015). The spill impacted over 630 hectares of stream and river habitat as well as floodplain and upland areas, injuring birds, mammals, reptiles and other wildlife (USFWS 2015). Total petroleum hydrocarbon (TPH) concentrations in sediments following the Kalamazoo spill, the largest dilbit spill in history, ranged from 100 to greater than 6000 mg TPH/Kg (Fitzpatrick et al. 2012; USFWS 2015). Reported concentrations of petrogenic polycyclic aromatic hydrocarbons (PAHs) in Kalamazoo spill affected sediments ranged from 7 to 170 mg PAH/kg (Fitzpatrick et al. 2012; USFWS 2015).

Compared to conventional crude oils, dilbit spills are particularly complex because of the differential weathering and environmental fate of the bitumen and light petroleum components, greater concentrations of asphaltenes and other high molecular weight hydrocarbons, and seasonal changes in blend composition (Dew et al. 2015; Lee et al. 2015; NAS 2016). The lighter components of dilbit rapidly volatilize, increasing the density, viscosity and propensity of the free product oil to sink in lower salinity environments (Stoyanovich et al. 2019). Once associated with sediment, dilbit hydrocarbons can persist in aquatic habitats and pose substantial challenges to oil recovery (Walker 2016; NAS 2016). Understanding the risks to aquatic species has been hampered by a general lack of understanding of the toxicity of dilbits to benthic organisms and the lack of ecotoxicity benchmarks for comparing environmental exposures (e.g., Dew et al. 2015; Lee et al. 2015; NAS 2016).

In the current study, two representative weathered heavy crude dilbits were assessed in laboratory oiled sediment tests: (1) Western Canadian Select (WCS), a blend of bitumen, Alberta heavy crude oils, and synthetic oils and condensates; and (2) Cold Lake Blend (CLB), a blend of Cold Lake bitumen and natural gas condensates. Dilbit effects on the freshwater amphipod Hyalella azteca were determined in standard U.S. EPA 10-day growth and survival tests, and on the estuarine amphipod Leptocheirus plumulosus in 10-day survival tests (USEPA 1994, 2000). These two species were considered optimal for assessing the toxicity of sediment-associated dilbits to benthic organisms because standardized test methods are available for these amphipods, and they are highly exposed epibenthic detritivores that burrow into the top few millimeters of sediment (Simpson et al. 2016).

MATERIALS AND METHODS

Test oils, weathering, and sediment preparation

The test oils were CLB and WCS dilbits that were artificially weathered by nitrogen gas stripping to no change in volume (~20% reduction; Barron et al. 2018). The control and treatment sediments were the same clean environmental sediment material collected in Alachua County, Florida USA at the edge of a natural freshwater water pond. The sediments were collected in 19 L HDPE vessels using a stainless-steel shovel. The sediments were collected by first removing branches and leaf litter. The sediment was collected from the surface to 6 cm to achieve 30 to 40 percent organic material while minimizing, but not excluding, the natural sand present. The collected sediment was refrigerated then press sieved and wet sieved down to a particle size of less than 400 μm. During the wet sieving process, the interstitial water was replaced with the water type used for the sediment overlay in sediment tests with H. azteca (synthetic freshwater) or L. plumulosus (20 ppt synthetic seawater). Water exchange included multiple iterations of settling the sediments under refrigeration followed by removal of the overlay water and mixing in the appropriate water type with a drill mounted stainless-steel auger until the species-specific water quality was achieved.

Test concentrations of weathered CLB or WCS dilbit in sediment were generated by mixing oil and sediment to the desired maximum concentration determined in preliminary range finding tests. The highest treatment in definitive tests with H. azteca was 10 g CLB or WCS per kg (ww) sediment, and in definitive tests with L. plumulosus was 1 g CLB/kg or 1 g WCS/kg (ww) (Figure SI-1). To ensure homogeneity of the sediment-oil mixture, batches of pre-measured oil and sediment were delivered to a tared and re-tared stainless-steel paint can. The can was sealed with Teflon tape and placed in a compressed air powered shaker for two ten-minute intervals. The can was rotated in between mixing intervals. A portion of the oiled sediment was transferred to exposure vessels and the remaining portion was further mixed with an equal portion of control sediment in order to create the next, less concentrated, oiled sediment treatment. A sample of the highest test concentration and control sediment were collected for analysis of petroleum hydrocarbons.

Test organisms and test conditions

Test organisms were the freshwater amphipod H. azteca and the estuarine amphipod L. plumulosus. H. azteca was cultured at the testing laboratory (Hydrosphere; Alachua, FL USA) and fed and maintained before testing according to methods in USEPA (2000). L. plumulosus were commercially obtained (Aquatic Research Organisms, Hampton, NH) and fed and maintained before testing according to methods in USEPA (1994). Both organisms were fed a preparation of yeast, cereal leaves and flake fish food during culture. Ten-day H. azteca growth and survival tests were conducted according to USEPA (2000); organisms were fed during testing as summarized in Table SI-1. Ten-day L. plumulosus survival tests were conducted according to USEPA (1994) as summarized in Table SI-2. In accordance with USEPA (1994), growth of L. plumulosus was not assessed because organisms were not fed during testing. H. azteca test vessels (300 mL) were filled with 100 mL sediment and 175 mL overlay water. L. plumulosus test vessels (1 L) were filled with 175 mL sediment and 800 mL overlay water. The test vessels were placed in temperature-controlled water baths and overlay water was added with minimal sediment disturbance (Fig. S1–2). Test vessels were allowed to settle overnight before the introduction of test organisms. In accordance with EPA (2000) guidelines, overlay water was renewed twice per day in H. azteca tests and test vessels were not aerated. In accordance with EPA (1994) guidelines, overlay water was not renewed in L. plumulosus tests and test vessels were aerated to maintain dissolved oxygen levels.

Water quality measurements in toxicity tests included: (1) daily monitoring of overlay water temperature and dissolved oxygen (both species), and pH and salinity (L. plumulosus); (2) monitoring of overlay water on Day 0 and Day 10 for pH and ammonia (both species), and hardness, alkalinity, and conductivity (H. azteca); and (3) pore water ammonia on Day 0 (both species). On the test days 1 through 9, the test vessels were checked for any notable activity and any observed mortalities were recorded. Complete live counts of both species were performed only at test end because normal organisms were buried and could not be confirmed each day. Tests were terminated after 10 days by transferring portions of the sediment samples through a 400 μm screen until all organism were found and counted. H. azteca were rinsed with deionized water and transferred to tared aluminum weigh pans for dry weight determination.

Analytical chemistry

Sediment samples were analyzed on a wet weight (ww) basis for petroleum hydrocarbons, grain size distribution, and organic carbon content. Samples of control and highest treatment sediment were collected at test initiation and test end without dilution with overlay water following sifting for organismal removal. Sediment samples were frozen immediately and shipped on wet ice to the analytical laboratory for measurements of total petroleum hydrocarbons (TPH), polycyclic aromatic hydrocarbons (PAHs), alkanes, grain size, total organic carbon, and percent moisture. Total organic carbon and inorganic carbon were measured in triplicate on 50 to 100 mg wet sediment according using a Total Organic Analyzer (TOC-VCPH) with a SSM-5000A solid sample combustion module (Shimadzu Scientific Instruments, Somerset, NJ). Particle size distribution was measured 20 times by laser in-situ scattering and transmissometry (LISST) on 0.15 g sediment diluted 2000 fold with deionized using the small volume adapter of the LISST 200X (Sequoia Scientific, Inc., Bellevue, WA).

For analysis of petroleum hydrocarbons, aliquots of each sediment sample (5 g) were first dried with 75 g of anhydrous sodium sulfate for 1 hr, then extracted using a ASE 350 Accelerated Solvent Extractor (Dionex™, Thermo Scientific, Waltham, MA). Samples were spiked with a surrogate mix and extracted with dichloromethane:hexane (5:1). The solvent mixture was pumped into the sample cell, up to a pressure of 1500 psi and the oven temperature was maintained at 100 °C. Three static cycles and a rinse volume of 120% was used during the extraction. Extracts were concentrated to 1 mL under the flow of nitrogen at 30 °C for analysis by U.S. EPA Methods 8270 and 8015D. All samples were extracted and analyzed in triplicates and the standard deviationbetween the replicate samples averaged less than 10%. Methods used to quantify target analytes in solvent extracts were detailed in Barron et al. (2018). In brief, 45 PAH compounds with 2, 3 and 4 rings and their alkylated homologs were analyzed by gas chromotography with mass selective detection and single ion monitoring (GC/MS SIM). Normal aliphatics ranging in carbon number from 10 to 35 and the branched alkanes pristine and phytane were also analyzed by GC/MS SIM. TPH in solvent extracts were analyzed by gas chromatography with flame ionization detection (Barron et al. 2018).

Quality assurance and data analysis

All toxicity testing and hydrocarbon measurements were performed in accordance with quality assurance project plans and standard operating procedures. Analytical chemistry calibration standards and known additions of hydrocarbons were made with a solution traceable to the U.S. National Institute of Standards and Technology. The accuracy of the standard was verified against a certified second source. The method was validated by analysis of a standard reference material (SRM2779 – Gulf of Mexico Crude oil) and sediment samples fortified with known quantities of hydrocarbons (Lab Fortified Matrix and Lab Fortified Matrix duplicate – LFM/LFMD). The second source concentration was within ±25% of the certified value for all compounds in the standard mix, and the SRM recovery was within ±30% of the certified value for most compounds. The LFM/LFMD recovery was within ±25% for most compounds, and the relative percent difference (RPD) between the LFM/LFMD was less than 25%.

Reference toxicity tests were performed with H. azteca and L. plumulosus and showed normal organism sensitivity. All water quality measurements remained within acceptable ranges in USEPA (1994, 2000) test guidance. Test concentrations were determined from the mean measured TPH and PAH (mg/kg sediment ww) in controls and highest treatment at day 0 and day 10. Trace artifactual concentrations of hydrocarbons measured in laboratory blanks were subtracted from each test concentration prior to data analysis. Test concentrations were normalized to one percent organic carbon using the measured OC concentrations in the freshwater (H. azteca) or saltwater (L. plumulosus) control sediments. TPH and PAH concentrations in other treatment levels were then extrapolated from the percent sediment dilution (i.e., 50%, 25%, 12.5%, 6.25%). Effect concentrations of TPH and PAHs were based on toxicity endpoints of survival (both species) and growth as final dry organism weight (H. azteca). All statistical analyses were performed using the R statistical platform (v. 3.3.3) and associated packages (R Development Core Team 2018; Ritz et al. 2015). Effect concentrations were all based on wet weight and included 10-day median effective concentrations (EC50), 20% effect concentration (EC20), no observed (NOEC) and lowest observed (LOEC) effect concentrations.

RESULTS

Dilbit and sediment chemistry

Laboratory weathering of the CLB and WCS dilbits increased total PAHs by 17.4 and 24.4% in the source oils, respectively, and resulted in almost complete loss of BTEX and no appreciable effect on total alkane concentrations (Table SI-3). The C2 and C3 homologs of napthalene, phenanthrene, fluorene and dibenzothiophene were the predominant PAHs in both weathered dilbits, and C11-C17 and C30 were the predominant normal alkanes (Fig. SI-3). The sediment used as controls and in dilbit treatments was dominated by grain sizes of 5 to 150 microns and contained 4.5% to 5.5% OC (Table SI-4). Moisture content of freshwater and saltwater sediments averaged 39.4% ± 0.3% and 43.8 ± 0.2%, respectively. Control sediments contained trace levels of PAHs and contained 4 to 5 mg/kg of C29, C31 and C33 normal alkanes (Fig. SI-4). TPH concentrations measured in test sediments ranged from 82% to 95% of nominal oiling levels, with PAH and alkane content proportional to sediment treatment (Table 1). Two and three ring PAHs predominated in sediment treatments with CLB and WCS dilbits and there were no appreciable changes in PAH composition during the 10-day exposures (Fig. 1). The predominate aliphatics in dilbit oiled sediments were C14 to C17, C29 and C31 normal alkanes and there were no appreciable changes in alkane composition during the 10-day exposures (Fig. 2).

Table 1.

Mean measured concentrations (wet weight) of TPH, total PAHs, and alkanes (wet weight) in dilbit oiled sediment tests with Hyalella azteca and Leptocheirus plumulosus.

Test Species Nominal oil (g/kg) Cold Lake Blend Dilbit Western Canadian Select Dilbit
TPH
(mg/kg)		 PAH
(mg/kg)		 Alkanes
(mg/kg)		 TPH
(mg/kg)		 PAH
(mg/kg)		 Alkanes
(mg/kg)
Hyalella azteca 10 9450 95.5 47.7 9120 97.6 64.0
Control 90 0.27 2.85 80 0.31 2.67
Leptocheirus plumulosus 1.0 820 8.39 8.00 820 9.59 9.68
Control 70 0.28 3.98 120 0.25 3.47
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Figure 1.

Figure 1.

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Composition (mg/kg ww) of normal alkanes (top panels; carbon chain C10 to C35) and PAHs and their alkyl homologs (bottom panels) in test sediment at Day 0 and Day 10 of Hyalella azteca exposures to Cold Lake Blend (CLB) and Western Canadian Select (WCS) dilbits (10 g oil/kg). NAP: napthalenes; PHE: phenanthrenes; FLU: fluorenes; DBT: dibenzothiophenes; NBT: napthobenzothiophenes; PYR: pyrenes; CHY: chrysenes.

Figure 2.

Figure 2.

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Composition (mg/kg ww) of normal alkanes (top panels; carbon chains C10 to C35) and PAHs (bottom panels) in test sediment at Day 0 and Day 10 of Leptocheirus plumulosus exposures to Cold Lake Blend (CLB) and Western Canadian Select (WCS) dilbits (1 g oil/kg). NAP: napthalenes; PHE: phenanthrenes; FLU: fluorenes; DBT: dibenzothiophenes; NBT: napthobenzothiophenes; PYR: pyrenes; CHY: chrysenes.

Toxicity of oiled sediments

All sediment tests met quality control criteria, including water quality values within acceptable limits, control survival greater than 95%, and measurable growth of H. azteca controls (240% to 280% increase in dry weight). L. plumulosus were more sensitive than H. azteca, and growth of H. azteca was a more sensitive endpoint than survival (Table 2). Dilbit EC20s and LOECs for TPH ranged from 216 to 1165 mg/kg sediment (ww, 1% OC) in H. azteca, and from 64 to 75 mg/kg sediment in L. plumulosus (Table 2). Dilbit EC20s and LOECs for total PAHs ranged from 2.9 to 11.8 mg/kg sediment (ww, 1% OC) in H. azteca, and from 0.75 to 0.87 mg/kg in L. plumulosus (Table 2).

Table 2.

Summary of no effect and effect concentrations of TPH and total PAHs in two species of amphipods exposed to dilbit oiled sediment. All values mg/kg wet weight, normalized to 1% sediment organic carbon.

Dilbit Test Species Endpoint NOEC LOEC EC20 (SD)A EC50 (SD)A
TPH PAH TPH PAH TPH PAH TPH PAH
Cold Lake Blend Hyalella azteca Survival 520 5.27 1042 10.5 1165 (108) 11.8 (1.08) 2445 (161) 24.7 (1.63)
Growth 260 2.62 520 5.27 216 (152) 2.89 (1.35) 1778 (491) 19.7 (4.76)
Leptocheirus plumulosus Survival 38.3B 0.38 74.7B 0.77 72.9 (1.82) 0.75 (0.02) 83.8 (12.8) 0.87 (0.11)
Western Canadian Select Hyalella azteca Survival 502 5.38 1005 10.8 789 (167) 8.46 (1.78) 1251 (148) 13.4 >(1.58)
Growth 502 5.38 1005 10.8 370 (108) 4.08 (1.10) 1082 (196) 11.7 (2.09)
Leptocheirus plumulosus Survival 38.3B 0.44 74.7B 0.87 63.8 (5.5) 0.75 (0.06) 104 (5.47) 1.23 (0.06)
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A.

SD: standard deviation of EC20 and EC50 values in parentheses.

B.

Interpolated value.

DISCUSSION

The key uncertainties in the assessment of risks and impacts of dilbit spills in aquatic environments continue to be differential weathering of component hydrocarbon blends, the fate of denser than water non-aqueous phase liquid, and the fate and effects of hydrocarbon associations with suspended and bedded sediments (Dew et al. 2015; Fitzpatrick et al. 2015; Lee et al. 2015; NAS 2016; Alsaadi et al. 2018; Hua et al. 2018; Stoyanovich et al. 2019; Johannessen et al. 2020). Lighter components of dilbit can rapidly and extensively volatilize and disassociate, increasing the density, viscosity and propensity of the remaining free product to sink (Winter and Haddad 2014; NAS 2016; Hua et al. 2018; Stoyanovich et al. 2019). In the current study, artificial weathering was used to remove volatile compounds to simulate the initial environmental weathering that occurs before dilbits move to sediment. Dilbits can interact with sediment in marine environments, but may be of greatest concern in freshwater and estuary habitats because of decreased oil buoyancy in lower salinity water (e.g., Hua et al. 2018; Johannessen et al. 2020). Several days after a spill, residual dilbit interacts with particulates and organic matter in the water column and ultimately associates with bedded sediments (Walker et al. 2016). Formation of oil-particulate aggregates is a key process controlling dilbit transport and fate in aquatic environments and their complex behavior is still not well understood (Fitzpatrick et al. 2015). Once associated with sediments, dilbits may persist and provide complex challenges for spill response and oil recovery (e.g., USEPA 2016; Fitzpatrick et al. 2015; Lee et al. 2015; USFWS 2015; NAS 2016; Walker et al. 2016). In the current study, the concentration and composition of TPH, PAHs and alkanes remained stable over the 10-day duration of the laboratory sediment exposures. The C2 to C4 alkyl homologs of the two and three ring PAHs predominated in all directly oiled sediments, and showed no relative changes compared to the weathered source oils.

While the knowledgebase on the toxicity of dilbits to water column organisms continues to grow, dose-response studies of the toxicity of dilbit oiled sediment to benthic organisms has not been previously reported (Dew et al. 2015; Barron et al. 2018; Alsaadi et al. 2018). Laboratory tests of water accommodated fractions of dilbits generally show similar toxicity as conventional oils, despite higher proportions of alkylated PAHs (e.g., Dew et al. 2015; Alsaadi et al. 2018; Barron et al. 2018). The current study utilized freshwater and estuarine amphipods as sediment test species because they are likely to be highly exposed to dilbit spills in aquatic habitats as epibenthic detritivores that burrow into the top few millimeters of sediment. Toxicity results were derived using artificially weathered dilbits that removed volatile organic compounds, but that did not incorporate additional environmental weathering processes such as biodegradation and photooxidation. An additional source of uncertainty was quantification of petroleum hydrocarbons in test sediments. Underestimation or overestimation of alkylated PAH concentrations can occur using the standard petroleum chemistry methods that are routinely used in oil impact studies (Wilton et al. 2017).

Despite physical-chemical differences in CLB and WCS (Polaris 2013; Barron et al. 2018), within-species toxicity values were similar. For example, 10-day median lethal levels (wet weight, normalized to 1% OC) for H. azteca ranging from 1082 to 2442 mg/Kg TPH and 11.7 to 24.7 mg/kg total PAHs, and 83.8 to 104 mg/kg TPH and 0.87 to 1.2 mg/kg for L. plumulosus. L. plumulosus was consistently more sensitive than H. azteca in all dilbit tests, but it is unknown if these differences were due to differences between test methods or intrinsic species sensitivity. Renewal of water overlying test sediments is prescribed in H. azteca tests whereas no water renewal is specified for L. plumulosus (USEPA 1994, 2000; Sheahan and Fisher 2012). Both species are epibenthic, remain buried during sediment testing, and likely respire only pore water in the sediment tests. However, the overlying water renewals with H. azteca may have contributed to lower hydrocarbon exposures and thus less observed sensitivity. Hydrocarbons in overlying water were not measured in either test the current study, adding to uncertainties in possible exposure differences between L. plumulosus and H. azteca. To our knowledge, there have been no reported studies comparing the relative sensitivity of these two amphipods, thus it is unknown if the differences in species sensitivity observed with the two dilbits is consistent for other petroleum products. Although epibenthic amphipods are routinely used as test organisms (Simpson et al. 2016), other invertebrate species may be more highly exposed to sediment and less confounded with overlying water exposures.

Laboratory-based toxicity values are routinely used to derive screening level benchmarks for assessing potential environmental effects of oil and other hazardous material releases (Jones et al. 1996; Barron and Wharton 2005; Hook 2020). Toxicity-based laboratory sediment benchmarks can be derived from the current study using ranges of NOEC and low effect (LOEC, EC20) values for the most sensitive species and endpoint tested. Based on the test results for L. plumulosus, potential toxicity-based benchmarks for dilbits would range from 210 to 410 mg oil per Kg sediment and 64 to 75 mg TPH/kg (ww, 1% OC). Benchmarks for total petrogenic PAHs would range from 0.75 to 0.87 mg/kg sediment (ww, 1% OC). These toxicity-based benchmarks are on the low end of the broad range of PAH benchmarks ( 0.26 to 100 mg/kg; Buckman 2008). Reports listing environmental concentrations of oil and PAH concentrations following dilbit spills are extremely limited. The July 2010 spill of CLB and WCS into the Kalamazoo River Michigan was the largest dilbit spill in U.S. history (Winter and Haddad 2014; USEPA 2016). Concentrations of dilbit in sediment were reported to range from 100 to 6000 mg/kg TPH and 7 to 170 mg/kg PAH, with some heavily oiled sites with likely acute and chronic risks to benthic fauna (Fitzpatrick et al. 2012; USFWS 2015; USEPA 2016). Comparison of reported TPH and total PAHs from the Kalamazoo spill with L. plumulosus-based benchmarks from the current study also indicate past risks to benthic organisms, with some samples even exceeding median lethal levels for the less sensitive H. azteca. Maximum reported concentrations of TPH (12,500 mg/kg) and PAHs (30.3 mg/kg) in sediments following the July 2007 spill of dilbit in Burnaby, British Columbia, Canada (Stantec 2012) also exceeded lethal concentrations and sublethal toxicity benchmarks determined for L. plumulosus in the current study. The applicability of benchmarks based on 10-day laboratory tests to environmental exposures is uncertain, but suggest the potential for dilbit spills to substantially impact benthic organisms as has been previously noted (e.g., Dew et al. 2015; Fitzpatrick et al. 2012; NAS 2016; Alsaadi et al. 2018; Stoyanovich et al. 2019). Sediment tests were performed according to published guidance with standard test species, and application to specific dilbit spills ultimately requires consideration of site-specific species, habitats, and environmental conditions.

CONCLUSIONS

The potential for dilbit spills remains a significant environmental concern because of increasing transport within North America and challenges to environmental cleanup exacerbated by uncertainties in the transport, fate and effects in aquatic habitats. Research needs and knowledge gaps identified by Lee et al. (2015) and NAS (2016) in comprehensive assessments of the behavior and impacts of dilbits included understanding oil-particulate aggregates and dilbit emulsification, factors affecting weathering and sinking of dilbits, and species-specific exposures and toxicity. Field studies using water column and shoreline mesocosms are beginning to elucidate some of the complex physical-chemical and trophic interactions and effects of dilbits in lotic systems (e.g., Stoyanovich et al. 2019; Cederwall et al. 2020). Comparison of toxicity-based benchmarks derived from the current study to reported sediment concentrations indicate that dilbit spills in aquatic habitats may pose substantial risks to freshwater and estuarine benthic organisms.

Supplementary Material

Sup1

Acknowledgements

We thank Vince Palace for comments on a draft of this manuscript. Pegasus Technical Services, Inc. and Hydrosphere Research were contractors to the U.S. EPA. The views expressed in this article are those of the authors and do not necessarily reflect the opinions or policies of the U.S. EPA. This publication does not constitute an endorsement of any commercial product. Data from this study are available through EPA’s Science Hub portal (https://sciencehub.epa.gov/sciencehub/datasets/SciIDA-7wmh).

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