FIGURE 4-5 Factors determining whether spilled oil will float or sink (NRC, 1999).

the process or the potential for the process to occur in a spill situation. Recent work on oil-fines interaction stimulated new research on the interaction of oil and suspended solids in glacier-fed rivers in Alaska. In laboratory mixing tests with water from eight rivers collected over three seasons in Alaska, oil-sediment interactions occurred, but the factors controlling the amount of oil loading could not be identified (McCourt and Shier, 2001).


Overwashing is the temporary submergence of oil below the water surface. The oil can be described as “floating” just below the water surface. Environment Canada conducted several studies in the 1980s to investigate the factors influencing oil submergence as part of an overall program on the behavior of spilled oil (Wilson et al., 1986; Clark et al., 1987; Lee et al., 1989). Equations for overwashing were developed by Mackay et al. (1986) and have been used in some models to predict this process. There have been no significant advances in the theory of overwashing since this work.

The principal cause of overwashing is the action of waves and near-surface turbulence (Clark et al., 1987). Two other factors are also very important: the density of the oil must be close to that of water and the oil must become viscous enough so that the slick breaks up into discrete masses such as tarballs (Buist and Potter, 1987).

Overwashing is particularly important because submerged oil is difficult to see visually or with remote sensors, making it difficult to detect the oil, track its path, and make accurate trajectory predictions. During the Nestucca spill, the oil broke into tarballs that became overwashed and could not be visually tracked. Two weeks later, the oil showed up along 150 km of shoreline on Vancouver Island, Canada (NOAA, 1992). Submerged oil also weathers more slowly because there is almost always a thin water layer on top of the oil (Clark et al., 1987). Thus, relatively fresh oil can travel hundreds of kilometers from the release site. Finally, it is very difficult to recover submerged oil using standard skimming equipment.

There are three mechanisms by which submerged oil can resurface: (1) the density of the water increases, as in an estuary where the oil moves from fresh water to salt water; (2) the turbulence of the water surface ceases, when the wind dies down or a river plume enters a bay; and (3) the oil becomes stranded on a shoreline.

Partitioning and Bioavailability

Petroleum hydrocarbons come in many chemical forms or species and partition among the myriad particulate and

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