Of wastes dumped deliberately into the sea, as much as 80% consist of 'dredge spoils', the materials scraped from river and harbour bottoms to open channels and facilitate navigation. These materials are natural, for the most part, made up of sand, silt, clay and rock. They are usually barged to offshore areas and dumped, often grossly disrupting these undersea locations for a period of time. These 'spoils' often include a high proportion of deposited sewage sludge and accumulated industrial waste, with its potential toxicity.
Although about half of the oil transported in the world each year is carried by tankers, and tanker oil is dumped as part of routine operations, (such as illegal deballasting and tank washing, which flush oil into the sea), most of the oil in the sea comes from a combination of land-based industrial and municipal sources. As much as 350 million gallons (1.3 billion litres) of used automobile crankcase oil is dumped each year into water drainage systems that reach the sea. About 600,000 tonnes of oil run into the sea each year a result of petroleum carried into the atmosphere from poorly tuned automobile engines. Of the twenty largest metropolitan areas in the world, sixteen are coastal cities, or cities on rivers which empty relatively quickly into the ocean. About 30 billion gallons (113 billion litres) of industrial and municipal wastes are discharged into the coastal areas of the USA alone every year.
The chief danger of ocean sewage dumping lies in the spread of viral and bacterial diseases - directly to bathers, indirectly through fishery products. On the basis of the toxicity and the volume of many wastes entering the sea it seem likely that sea life could be seriously disrupted in some places more than others, but the threat is global.
Because the sea and its organisms can handle this load of natural wastes quite well, the misconception has arisen that there is no limit to what the sea can dispose of in its self-purification systems. The human waste from a large city like Los Angeles is greatly concentrated in one location, not dispersed widely, and it is usually spiked with toxic substances that the sea may be unable to recycle naturally. There is natural seepage of petroleum into the sea from ocean floor deposits, but human-caused oil pollution is now about ten times that rising naturally from the ocean bottom. Perhaps the most pernicious misconception about the sanitation system of the ocean, however, is that coastal dumping is a negligible contribution to an environment so vast and deep, covering nearly three quarters of the planet's surface. The fact is that most of the sea's life is concentrated along the world's coastlines, where nutrient-rich currents pass across convenient hiding and attachment surfaces shallow enough to be touched by sunlight. Of the 140 million square miles (363 million square kilometres) of ocean surface, it is only the 14 million square miles (36 million square kilometres) near shore that contain the sea's most important living habitats. About 90% of the world's food fish spawn, mature, and are caught, in these coastal areas. Unfortunately, the land adjacent to these productive waters is and where human populations gather, where many industries locate, where the greatest mass of destructive waste matter is dumped.