Oil Spill Containment and Cleanup Technology
A variety of oil spill response technologies are available, many of which were either tested on a small scale or utilized on a large scale in the Exxon Valdez oil spill. The cost of the cleanup, which was suspended during the fourth summer season in 1992 with Coast Guard approval, exceeded $2 billion, most of it assumed by Exxon. Assessment of the relative merits of a variety of oil spill cleanup methods, however, is complicated by the physical variables that affect oil cleanup: time, oil composition, location of spill, and weather. Because oil spreads rapidly once spilled, the longer a spill spreads, the more equipment is required for the cleanup. This factor was certainly critical to the extent and subsequent cost of the Exxon Valdez oil spill cleanup. Oil composition influences cleanup because higher viscosity oil is more difficult to recover due to its tendency to clog mechanical pumps. The effect of weather is twofold: rough seas facilitate the formation of an intractable oil-water emulsion known as "mousse" and adverse weather complicates the deployment and operation of cleanup equipment. Finally, an offshore oil spill, especially in a more remote location, exacerbates logistics problems related to equipment availability. As the U.S. Office of Technology Assessment (OTA) has concluded, "the speed of a response is critical and is dependent on rapid decision-making, logistics, and training." 6
The extent of oil evaporation is a function of the oil composition, the area and thickness of the oil slick, the temperature, and wind speed. Distillation fractions representing approximately 20% of Prudhoe Bay (North Slope) crude oil are most likely to evaporate under environmental conditions. Less volatile (higher boiling) components might evaporate, but only in the absence of the competing processes of dispersion and biodegradation.
Using a sophisticated computer simulation program, the On-Scene Spill Model (OSSM), scientists at the National Oceanographic and Atmospheric Administration (NOAA) have attempted to account for the fate of oil spilled from the Exxon Valdez. Mass balance estimates generated from this model predict that 20% of the original spilled oil evaporated. Once evaporated, the petroleum hydrocarbons are rapidly oxidized to photolysisj products. The more toxic monoaromatic and naphthalenic components , with a half-life in air of ca. 1 day, were 99% degraded within one week. While dispersion, according to this model, accounted for as much as 23% of the spilled oil just two months after the spill, after three years less than 1% remained in the water column (except as biodegradation products) and 13% resided in subtidal sediments, mainly in the Gulf of Alaska. Although oil is less dense than water, the adsorption of oil onto suspended particulate matter creates the possibility that adhered oil can sink to the bottom. The remainder of the dispersed oil was subsequently beached or biodegraded in the water column.7 The fate of the spilled oil at three different junctures - the first week of the spill (March 1989), three months after (June 1989), and three years later (June 1992) - is summarized in Table 5.
|Process||March 1989||June 1989||June 1992|
|Evaporation and photolysis||10%||18%||20%|
|Dispersion (water column)||4%||28%||38%|
* 50% biodegraded on beaches & in water column
13% settled in subtidal sediment
6% recovered from beaches during cleanup
2% weathered on intertidal shoreline
1% remained in water column
Oil spill containment and cleanup devices consist of booms and skimmers, a technology that has seen marginal improvement over the past two decades. Booms are long, tube-like barriers equipped with underwater skirts that act as floating fences to contain the oil spill so that skimmers can be employed to pump the surface oil-water mixture and separate the oil from the water. If efficient, mechanical methods offer a benign cleanup strategy. However, using the current technology, typically only 10% or less of oil from major oil spills has been recovered. The most recent advancement in this technology has been the design of large dual-purpose vessels developed in the Netherlands as hopper dredges to keep the port open and as skimming vessels to assist in oil cleanup.8
This technology was, in fact, employed in the cleanup of the Exxon Valdez oil spill. However, its sole use was recognized as woefully inadequate. Even if all the containment boom in the U.S. Navy inventory had been deployed to the spill within the first 12 hours, it would have been barely sufficient to encircle the spill. What is more incredulous is the fact that the U.S. Navy response was not even requested until more than one week after the Exxon Valdez accident. Recovery methods ultimately accounted for over 8% of the original oil spill according to the OSSM computer model.7
Dispersants are detergents that break up oil slicks into small droplets that disperse into the water column, where they are subjected to gradual biodegradation by natural processes, or that sink into bottom sediments. The surface-active agents (surfactants) stabilize the oil droplets by orienting in the oil-water interface with the hydrophobic (lipophilic) end of the surfactant molecule in the oil phase and the hydrophilic end in the water phase (Figure 4). Most chemical dispersant formulations also contain a solvent to reduce viscosity and to facilitate dispersal. The advantage of dispersants is that they can be spread rapidly over a large area and can reduce the amount of shoreline oil contamination.
Figure 4. Mechanism of Chemical Dispersion
A: Surfactant locates at oil-water interface.
B: Oil slick is readily dispersed into micelles or surfactant-stabilized droplets with minimal energy.
Source: Reference 9, p 29.
The use of chemical dispersants on an oil spill is controversial due to perceived toxicity problems. Depending on the crude oil source, 20-35% of a fresh oil spill can evaporate in the first day or so after the spill. However, when chemical dispersants are used to disperse oil into the water column, the hydrocarbons that preferentially dissolve are the slightly more water-soluble aromatic hydrocarbons, the toxicity of which we have mentioned previously. Benzene, for example, is a human carcinogen, and acute exposure can result in respiratory failure. Thus, more marine organisms could be affected by toxic components of dispersed oil in the water column.
While early dispersants were toxic to many marine organisms, those that are currently available are less toxic than the oil they disperse. Much of the unfavorable notoriety of chemical dispersants stems from their use at the 5-million-gallon Torrey Canyon oil spill in Cornwall, England, in 1967. Due to use of dispersants, the algae, limpet, barnacle, and mussel populations were devastated. In fact, the acute toxicity of the dispersants used was attributed to the alkylphenol surfactants and the aromatic hydrocarbons in the solvent. With the reduced toxicity of newer formulations, major harm should not occur to biological species in near-surface water (other than impairment of insulation capability of fur and feathers).9
Controversy Surrounding Chemical Dispersants. The effectiveness of chemical dispersants, which depends on sea conditions and application techniques as well as the chemical nature of both the dispersants and the oil, is now of more concern than dispersant toxicity. EPA had devised a contingency plan that would allow each state to decide if and when dispersants would be used. The state of Alaska had thus established three dispersant-use zones. While dispersant use was not recommended at all in a zone 3 area (a fragile ecosystem), EPA and the state of Alaska had given the Coast Guard on-scene coordinator (OSC) pre-approval to use dispersants in regions far from sensitive shorelines. designated zone 1 areas. Although Bligh Reef was designated a zone 3 area, within 24 hours of the spill the oil had begun to extend into the more open zone 1 waters of PWS. The request by Alyeska for a dispersant trial, sent at 8:00 am on March 25, was delayed by a faulty FAX machine at the Coast Guard station in Valdez. Approval for the dispersant trial by the Coast Guard OSC came at noon on March 26. At 4:00 pm on March 26, a plane loaded with 3700 gallons of dispersant took off from Valdez. Differences of opinion emerged about the efficacy of the trial; while Exxon's Iarossi viewed it as quite successful, the Coast Guard OSC equivocated due to the lack of wind and surface action. Harvard's Dr. James Butler of the National Academy of Sciences cautioned that "not using dispersants because the water is too calm is a fallacy."5 Finally, by 7:00 pm on Sunday, March 26, agreement was reached to use dispersant in zone 1. However, the amount of dispersant available in Alaska, 365 drums, would only have covered 10% of the oil spill. As Exxon began flying in dispersant from around the world and manufacturing more at two facilities, the spring blizzard that arrived early Monday morning with gale-force winds effectively halted dispersant use before it had begun.
Fresh oil, which still contains many volatile components, is readily ignited by incendiary bombs lowered from helicopters. Burning can, in principle, eliminate up to 90% of the oil, thus sparing marine life and beaches. However, combustion releases black sooty smoke that contains toxic gases, which might cause nausea, headaches, and respiratory problems. The U.S. OTA has determined that combustion products released into the atmosphere are not more hazardous than those released by evaporating oil.6
Before spilled oil can be subjected to a controlled burn, the oil must be cordoned off with fireproof boom. Although only 500 feet of fire boom was available in Alaska at the time of the spill, the arrival of an additional 2500 feet of fire boom by noon on Saturday, March 25, was sufficient to allow for a test burn. The test burn successfully burned off approximately 15,000 gallons of oil in less than one hour, leaving only a small pool of tarlike residue. As Exxon prepared to run four additional test burns, this effort was stymied by the requirement of a permit from DEC due to the smoke irritation experienced by residents of the nearby village of Tatilek. The delay was critical because as time passed the more flammable components of the oil were evaporating and the remaining oil was more inclined to develop into the water-oil emulsion or "mousse" that would resist burning. DEC opted not to provide the permit for additional controlled burns.5 In 1978, French farmers on the coast of Brittany effectively halted talk of igniting the wreck of the Amoco Cadiz because they feared that soot would ruin their crops. And in 1990, when the Mega Borg tanker caught fire fifty miles off the coast of Texas, the public virtually demanded that the fire be extinguished, which it was.