PHOTO 17 Oil on intertidal flat in Saudi Arabia in May 1991. The heavy oil slicks persisted long enough (for months) in protected bays to coat the entire intertidal zone. (Photo courtesy of Jacqui Michel, Research Planning, Inc.)

solved hydrocarbons, operationally defined as passing through a 0.2-μm filter and including colloids and small particles. Any geochemical process that decreases the dissolved hydrocarbon concentration, including partitioning onto larger solids, will reduce hydrocarbon exposure of phytoplankton. Conversely, partitioning of hydrocarbons onto organic-rich particles, including plankton and detritus, results in transfer of hydrocarbons to higher trophic levels, including filter feeders (e.g. mussels, oysters), fish, and mammals. The extent of hydrocarbon accumulation in organisms is controlled by the desorption rate of hydrocarbons from particles in the gut (for higher trophic organisms) and metabolic rates that degrade or transfer hydrocarbons outside the cell.

Shoreline Stranding and Tarball Formation

Persistent oil residues have two major fates: shoreline stranding for spills near to shore and tarball formation for releases in offshore waters. Oil loading on a shoreline can be highly variable, and the amount of oil and the rate of natural removal drive the decision to conduct shoreline cleanup. The sensitivity of shorelines to oil has been embodied in a ranking system called the Environmental Sensitivity Index (ESI) that has been widely applied for oil spill planning and cleanup decision-making (Halls et al., 1997; Hayes et al., 1980); (see Box 2-2). The ESI classifies and ranks shorelines according to the factors that influence oil persistence and impacts, such as degree of exposure, substrate permeability, and shoreline slope. Highest on the scale are the sheltered habitats, such as muddy tidal flats, marshes, and mangroves. These shoreline types are usually priority areas for protection because of their sensitivity and the difficulty of cleanup. Gravel beaches have the highest ranking for beaches because their high permeability allows deep penetration, complex patterns for sediment reworking during storms, low rates of natural replenishment, and the presence of localized, sheltered areas where oil can persist for years (Hayes and Michel, 2001). Decision-making for shoreline cleanup must evaluate the trade-offs between the impact of the oil and the impact of the cleanup. The objective is oil removal to the point that allows recovery without causing more harm than leaving the oil in place (NOAA, 2000). There are well-established guidelines for shoreline assessment, cleanup methods, and cleanup end points (NOAA, 2000; Environment Canada, 2000).

A coastal zone oil spill model (COZOIL) was developed to predict the behavior, loading, and fate of oil stranded on different shoreline types (Reed and Gundlach, 1989). It considers the oil density, viscosity, wave energy, grain size of



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