Coastal Processes FAQ - What are tar balls?

Most particulate petroleum residues in the sea are found floating at or near the water surface and are commonly called “tar balls.” They are the residues that remain after various physical and chemical processes have acted on floating oil for varying periods of time. Scientific studies have estimated that approximately 10-30 percent of oil discharged to the ocean remains in the form of tar balls and may persist for approximately one year.

The size of tar balls can range from very small (less than 1 millimeter) to many centimeters in diameter, although most are quite small. Tar balls found near the shore or in the surf zone area are often broken into smaller pieces. Chemical compositions of tar balls are extremely variable, and the physical appearance can range from brittle and hard to soft and very sticky.

The ultimate fate of most tar balls at sea probably consists of break-up and sinking within a period of one year. Tar ball sitings on beaches have been reported globally. The decomposition of beach-stranded tar balls is extremely dependent on the shoreline substrate, the energy regime of the shoreline environment, the presence or absence of nutrients, and the rate of sedimentation within the intertidal zone.

Tar ball concentrations and compositions vary throughout the world’s oceans, but localized “hot spots” of tar ball accumulations have been reported in several areas. Generally, the Mediterranean Sea and the Sargasso Sea have high levels of tar ball occurrence which are greater than levels observed in the North Atlantic and North Pacific oceans.

Studies of the 1989 Presidente Rivera oil spill in the Delaware River found that the spilled oil (No. 6 fuel oil) was very dense and formed clumps with diameters of up to 10 feet. Because of the oil’s density relative to water (specific gravity = 0.99), the oil behaved unpredictably – some of it floating and some of it sinking to the bottom, as evidenced by the presence of bottom debris in tar balls which were recovered at the water’s surface.