The statement in the Georges Bank Study () that three-quarters of the oil spilled in accidents comes from the 25 percent of accidents due to technical failure does not include the pollution caused by pumping ballast water overboard. This is because the study was concerned only with accidents, whereas the pumping of ballast water is "intentional." The deballasting of tankers accounts for approximately 70 percent of the tanker-caused oil pollution of the oceans today (). Therefore, with the possible exception of pollution from highway motor vehicles, ballast water discharge is the largest source of unnatural influx of oil into the sea.

Oil tankers usually carry products in only one direction and return voyages are made without cargo. For a ship with the size and weight distribution of an oil tanker, going to sea unloaded is neither comfortable nor safe. For this reason, these ships fill some of their tanks with sea water (called ballast water) when making return voyages. The oil remaining in the bottoms of the tanks and adhering to the walls of the tanks mixes with the ballast water on these voyages and, unless special equipment and procedures are used, the oily water is discharged directly into the sea prior to the tanker's arrival at port. The first two items in Table 1-1 relate to the discharge of oily ballast, the first relating to tankers using a pollution-minimizing procedure called "load-on-top," and the second relating to tankers not using this procedure. The deballasting of tankers usually takes place about twenty or thirty miles offshore, and much of the associated oil never reaches shore, so the coastline is far less polluted by ballast water discharge than is the ocean itself.

New standards for high seas deballasting were recently established by the Inter-Governmental Maritime Consultative Organization (IMCO). If ratified by the necessary fifteen nations, the standards will require all ships over 150