Supplement 2.5: Weathering of Oil on the Water Surface (2/4)
- Physical Processes -

4. Sinking / Sedimentation

Photo: ITOPF

Most oils have a density less than that of sea water (see drop down box below). Therefore they don't tend to sink. However, sinking occurs due to the adhesion of particles of sediment or organic matter to the oil. Especially in shallow coastal waters solid particles, such as sand or algae, are abundant providing favourable conditions for sedimentation. Oil that has been washed on the shorelines often becomes mixed with sand. If this mixture is subsequently washed off the beach and back into the sea its density is large enough for it to sink.

Oil covers a beach on the Tuzla Spit in the Kerch Strait which connects the Black Sea with the Sea of Azov. Soldiers pitch in to clean up the spill.
Photo: Russian Doors / Marine Photobank

5. Dissolution

The rate and extent to which an oil dissolves in sea water depends upon its composition, spreading, water temperature, turbulence and degree of dispersion. The heavy components of crude oil are virtually insoluble in sea water whereas lighter compounds are slightly soluble. However these compounds are lost rapidly through evaporation. So dissolution does not make an important contribution to the oil left on the sea surface after a spill.

Dissolution of motor oil in water. In the left bottle the oil floats on the water. After approximately 5 minutes of shaking an emulsion has formed (see next page). The milky colour of the water indicates that parts of the oil have dissolved in the water.
Experiment and Photo: Matthias Zech