FIGURE 4-2 Evaporation rates of different types of oil at 15ºC (adapted from Fingas, 2000).

lower in the water column, showing different drag with respect to the wind. Oils will generally take up water once spilled at sea, but emulsions may not always form. Water can be simply “entrained” by the oil due to viscous forces, without forming a more stable emulsion. Thus, emulsification also has significant effects on the choice of oil spill recovery methods.

In the late 1960s, Berridge et al. (1968) were the first to describe emulsification in detail. They measured several physical properties and described the emulsions as forming because of the presence of asphaltenes and resins. After these studies, there was little emphasis on the causes of emulsion formation. Mackay et al. (1982) hypothesized that emulsion stability was due to the formation of a film in oil that resisted the coalescence of water droplets; however, this work was used largely for modeling, and not for understanding, the process. Several studies have shown that water is stabilized in oil by two forces: viscous and elastic forces resulting from the interfacial action of resins and asphaltenes. This stabilization was noted as early as the 1970s when formation of emulsion correlated with the oil (Fingas et al., 1996). Only in the 1990s did studies show the effects of composition and propose clear reasons for water-in-oil emulsions. A significant factor in defining mechanisms and other characteristics of emulsions has been the development of analysis techniques for them. Sjöblom et al. (1999) have been instrumental in conducting studies on the formation of emulsions, focusing on the emulsions associated with oil production. Methods were developed to use radio-frequency conductivity to study emulsions. This group also used a Langmuir trough to show that asphaltenes formed barriers of greater strength than those formed by resins.

BOX 4-1 T/B North Cape Barge Spill, Rhode Island

On January 19, 1996, the tug Scandia caught fire while towing the tank barge North Cape. The tug and tow were abandoned and storm-force winds pushed the Scandia ashore at the Trustom Pond National Wildlife Refuge, 3 miles west of Pt. Judith, Rhode Island. Approximately 2,850 tonnes (828,000 gallons) of home heating oil were released from the barge over a two-day period (Michel et al., 1997). Oil spread over a large area and throughout the water column, resulting in a fishing closure for approximately 250 square miles of Block Island Sound and seven coastal ponds that lasted nearly five months (Mauseth et al., 1997). This spill highlighted the conditions that occur when a light oil is released under high-wave-energy conditions, resulting in very high loading of a refined oil directly into the water column immediately after release. Thus, there was little time for traditional oil weathering processes to occur, whereby the toxicity of the oil is reduced by evaporation of the lighter, more toxic components. Two types of home heating oil were spilled, containing 3 and 6 percent polynuclear aromatic hydrocarbons (PAH), dominated by the 2- and 3-ringed PAH. An estimated 80 percent of the initial release of 700,000 gallons during the storm was physically dispersed into the water column and 12 percent evaporated in the first eight hours after each discharge. Only 10 percent of the oil was estimated to remain on the water surface in the form of sheens after the first 24 hours. Dissolved and dispersed oil concentrations in the water column reached 1-6 parts per million (ppm) total PAH. The dispersed oil droplets resurfaced during calm periods, leaving the dissolved fraction behind. The plume of dissolved oil moved along and offshore, significantly affecting benthic resources. In contrast, very little oil stranded on the shoreline, with relatively small impacts on marshes and intertidal communities, and no shoreline cleanup was necessary.

Nearshore benthic resources were greatly impacted, with estimated mortality of 9 million lobsters (mostly juveniles), 19.4 million surf clams, 7.6 million rock and hermit crabs, 4.2 million fish, and 2.8 million kilograms of amphipods and worms (NOAA et al., 1998). The extent of impacts to benthic resources was a function of the richness of the nearshore habitat, particularly for juvenile lobsters and surf clams, as well as the very cold conditions during the spill (water temperatures were 4ºC). Acute mortality of benthic organisms in the salt ponds, particularly amphipods, was also estimated to be high. There were no population-level impacts on winter flounder adults who were present and spawning in the ponds at the time of maximum exposures, nor were there any growth or survival impacts for young-of-the-year winter flounder in the ponds that year.

Wildlife directly affected by the spill were seabirds and waterfowl in wintering grounds in nearshore marine waters. Of the 114 live birds collected, all but 9 died or were euthanized. The very cold conditions during the spill decreased the survival rate. Total bird mortality was estimated to be 2,300 birds (Sperduto et al., 1998). Piping plovers showed reduced productivity the breeding season after the spill (NOAA et al., 1998). The rapid weathering of the light refined oil and the absence of mixing with fine-grained sediment limited ecological impacts to short-term toxic effects on both water column and benthic resources.

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