The small amounts of residue resulting from in situ burning (ISB) of oil on water can cause environmental concerns. This is particularly true if the residue sinks. Results of laboratory tests suggest the possibility that burn residues may sink for about 40 to 60% of crude oils worldwide. However, whether results from laboratory tests can be extrapolated to large-scale spills is not known.

Burn residues have little to no acute aquatic toxicity. Their greatest impact would likely be to the benthos (life on the ocean floor) from smothering. For most ISB applications, impacts would be very localized because of the small volumes of residues generated and their dispersal by currents.

Residues of oils burned in laboratory tests in the 1970s and 1980s floated. This is probably because of the small scale of those tests and the thinness of the burned oil. The 1991 Haven spill and subsequent in situ burn near Italy created large amounts of heated and burned oil residue which sank, stimulating research into whether residue density affects whether a residue will float or sink.

Results from larger-scale laboratory and meso-scale field tests suggest that the most important factors determining whether an ISB residue will float or sink are:

1. Water density
   Burn residues that are denser than the receiving water are likely to sink. The density of fresh water is 0.997 g/cm³ at 25°C, and the density of sea water is 1.025 g/cm³.
2. Properties of the starting oil
   Correlations between the densities of laboratory-generated burn residues and oil properties predict that burn residues will sink in sea water when the burned oils have (a) an initial density greater than about 0.865 g/cm³ (or API gravity less than about 32°) or (b) a weight percent distillation residue (at >1000°F) greater than 18.6%. When these correlations are applied to 137 crude oils, 38% are predicted to sink in seawater, 20% may sink, and 42% will float.
3. Thickness of the oil slick
   Residues from burns of thick crude oil slicks are more likely to sink than residues from burns of thin slicks of the same crude oils, because higher-molecular weight compounds concentrate in the residue as the burn progresses.
4. Efficiency of the burn
   Factors affecting burn efficiency include original slick thickness, degree of emulsification and weathering, area coverage of the flame, wind speed, and wave choppiness. For efficient burns, removal efficiencies are expected to exceed 90% of the collected and ignited oil. Rules of thumb for predicting residue thickness are [1]:
   • For unemulsified crude oil up to 10-20 mm thick, residue will be about 1 mm thick.
   • Thicker slicks result in thicker residues (up to 3-6 mm thick).
   • Emulsified oils can produce much thicker residues.
   • For light/medium refined products, the residue will be about 1 mm thick, regardless of slick thickness.

When burn residues sink, they do so only after cooling. Models of cooling rates predict that ambient water temperature will be reached in less than 5 minutes for 3 mm-thick residues, and in 20-30 minutes for 7 mm-thick residues [2].

Physical properties of burn residues depend on burn efficiency and oil type. Efficient burns of heavy crudes generate brittle, solid residues (like peanut brittle). Residues from efficient burns of other crudes are described as semi-solid (like cold roofing tar). Inefficient burns generate mixtures of unburned oil, burned residues, and soot that are sticky, taffy-like, or semi-liquid.

Chemical analyses of burn residues show relative enrichment in metals and the higher-molecular weight PAHs, which have high chronic toxicity but are thought to have low bioavailability in the residue matrix. Bioassays with water from laboratory- and field-generated (Newfoundland Offshore Burn Experiment, or NOBE) burn residues of Alberta Sweet Mix Blend showed little or no acute toxicity to sand dollars (sperm cell fertilization, larvae, and cytogenetics), oyster larvae, and inland silversides [3]. Bioassays using NOBE burn residues showed no acute aquatic toxicity to fish (rainbow trout and three-spine stickleback) and sea urchin fertilization [4]. Bioassays using laboratory-generated Bass Strait crude burn residue showed no acute toxicity to amphipods and very low sublethal toxicity to marine snails (burying behavior) [5].

Localized smothering of benthic habitats and fouling of fish nets and pens may be the most significant concern when semi-solid or semi-liquid residues sink. At the 1983 Honan Jade spill in South Korea, burn residue sank in two hours and adversely affected nearby crab pens [6]. All residues, whether they floated or sank, could be ingested by fish, birds, mammals, and other organisms and may also foul gills, feathers, fur, or baleen. However, these impacts would be expected to be much less severe than those manifested through exposure to a large, uncontained oil spill.

In situ burning at the 2010 Deepwater Horizon/BP oil spill played a significant role in reducing the amount of oil on the water's surface. There were 411 burn operations conducted during the course of the response that removed an estimated 250,000 barrels of oil [7]. The in situ burn operation eventually grew to include three task forces, each consisting of a three vessel ignition team, two task force vessels, one supply vessel, a safety team, and five fire boom teams. The task forces were directed to targets by spotter aircraft.

The ISB Unit developed a logistically complex process to coordinate skimming teams, dispersant operations, and in situ burn teams, keeping each clear of the others. In situ burns did not result in any injuries, and air quality testing near the burns was conducted to ensure worker safety [8]. Special considerations were also given to wildlife, especially sea turtles, during these operations.

Because of uncertainties in extrapolating laboratory results to actual spill conditions, responders cannot confidently predict the amount of residue that may be generated by burning heavy crude oils and refined products or if/how much of the residue will float or sink.

Only a very short time window is available for surface recovery of residues that eventually sink, but this recovery option could be effective, since residues are readily recovered either manually or with sorbents. Limitations include logistics, worker safety, and slow-down in ISB operations. Residues may be re-burned as more oil is collected and burned. Once the residue sinks, recovery options are few, logistics-intensive, and ineffective.

Needed Research

Field trials and study of actual spills where ISB is conducted are needed to determine whether or not the small-scale test data and predictive models developed to date apply to large burns. These models then should be refined.

Chronic toxicity tests using burn residues, benthic organisms and habitats, and realistic exposure levels and pathways also are needed.

1. Buist, I. and K. Trudel. 1995. Laboratory studies of the properties of in-situ burn residues. Technical Report Series 95-010, Marine Spill Response Corporation, Washington, D.C., 110 pp.
2. S.L. Ross Environmental Research Ltd.. 1998. Identification of oils that produce non-buoyant in situ burning residues and methods for their recovery. American Petroleum Institute and the Texas General Land Office, Washington, D.C., 50 pp.
3. Daykin, M., Ga. Sergy, D. Aurand, G. Shigenaka, Z. Wang, and A. Tang. 1994. Aquatic toxicity resulting from in situ burning of oil-on-water. Proc. Seventeenth Arctic and Marine Oilspill Program Technical Seminar, Environment Canada, Ottawa, Ontario, pp. 1165-1193.
4. Blenkinsopp, S., G. Sergy, K. Doe, G. Wohlgeschaffen, K. Li, and M. Fingas. 1997. Evaluation of the toxicity of the weathered crude oil used at the Newfoundland Offshore Burn Experiment (NOBE) and the resultant burn residue. Proc. Twentieth Arctic and Marine Oilspill Program Technical Seminar, Environment Canada, Ottawa, Ontario, pp. 677-684.
5. Gulec, I. and D.A. Holdway. 1999. The toxicity of laboratory burned oil to the amphipod Allorchestes compressa and the snail Polinices conicus. Spill Science & Tech., V. 5, pp. 135-139.
6. Moller, T.H. 1992. Recent experience of oil sinking. Proc. Fifteenth Arctic and Marine Oilspill Program Technical Seminar, Environment Canada, Ottawa, Ontario, pp. 11-14.
7. On Scene Coordinator Report Deepwater Horizon Oil Spill. Submitted to the National Response Team, September 2011.
8. Screening Level Assessment of Risks Due to Dioxin Emissions from Burning Oil from the BP Deepwater Horizon Gulf of Mexico Spill [PDF, 255.4 KB]. John Schaum, Mark Cohen, Steven Perry, Richard Artz, Roland Draxler, Jeffrey B. Frithsen, David Heist, Matthew Lorber, and Linda Phillips. Environmental Science & Technology 2010 44 (24), 9383-9389.

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