Slow weathering rates further increase the risk of chronic exposures. Smothering and coating can be heavy. Bioavailability varies with oil and spill conditions. There is a high probability of chronic impacts from exposure to refloated oil and restranded tar balls on shores after storms. Oil in the water column is not likely to have an impact on highly mobile species.
Closures are likely to be longer.
Actual risks for a specific spill will be a function of the composition and properties of the spilled oil and environmental condi- tions at the spill site. Oil on the surface is primarily weathered by evaporation to the atmosphere and, to a lesser degree, to the water column by dissolution. Oils suspended in the water column or deposited on the bottom are less likely to evaporate but more likely to dissolve, although the water-soluble fraction of heavy oils is usually very low. Consequently, the water column can have higher concentrations of toxic fractions from nonfloating oils than from floating oils.